Grimnir's install is broadly considered meme-tier, but believe it or not, it DOES give several serious and unironic advantages that can warrant pressing, especially in matchups where U skills aren't as necessary. Before we get into the advantages of install, let's first discuss the advantages of Grimnir's other metered options.
236U (ult tornado): Premium anti-zoning option and solid get-off-me button at midrange. In the matchups where you want to keep threat of ult tornado alive to react to fullscreen fireballs, install might not be worth pressing.
214U (ult kick): Used in most of his midscreen combos. Frequently necessary to convert whiff punishes. Fullscreen antiair. Sometimes you can just throw it out and get lucky. All around a really good tool.
623U (ult DP): Grimnir's invincible reversal, and also his most reliable antiair. Goes without saying, but your defense is much stronger if you have meter to threaten this.
22U (ult sigils): Used in fuzzy setups all over the screen. Necessary for camera mix. Can run up and press it as a throw crush.
j.214U (ult airball): Mostly only used for tornado setup. (But tornado setup is strong, so that isn't nothing!)
Now let's look at how we can replicate these functions using install.
236U (ult tornado): There is no replacing this one with install. If you want to keep the fullscreen threat of ultimate fireball alive, you can't press install. This is the most glaring weakness.
214U (ult kick): Install gale dash replaces ult kick as your midscreen combo converter, whiff punish converter, and "throw it out and get lucky" tool. It isn't as good at antiairing, but honestly 214U is extremely inconsistent as an antiair already so it isn't much of a loss.
623U (ult DP): Install doesn't have a frame 1 invincible reversal, but opening up the ability to seal dash at any time on defense is very strong in its own way because airdash is one of the strongest counterpokes in the game. Beats 66L, most projectiles, and many other pressure resets clean. If you get desperate, you can always brave counter instead.
22U (ult sigils): You lose camera mix, but you can still do fuzzy and tornado setups while in install. Being able to throw bait from anywhere spikes your offense in a different direction - where your high/low gets weaker, your strike/throw gets stronger.
j.214U (ult airball): There's an alternate tornado setup you can do while in install, so losing this doesn't really hurt.
Super: Grimnir's other frame 1 invincible reversal. Being able to reaction super can be threatening in some matchups, but as mentioned above, install makes your defense good in other ways. Offensively, you lose the ability to strip BP, which is a notable downside; and you don't gain BP back for popping install for some reason, which is another notable downside.
So the main things you lose from popping install are the threat of ult fireball, your invincible reversals, and the ability to restore/strip BP. Everything else can be covered with install tech.
It also opens up new possibilities: you can throw bait from anywhere, fuzzy without seals, and move around in ways that are harder to predict. You can threaten airdash counterpoke from any position. And you can convert any hit into a corner fuzzy.
Install can be particularly worth pressing when the enemy is one hit from death, but super won't be enough to kill; or, when you get reset to neutral and one more confirm will win.
(starter) > 214H > 2M > 236H > (delay) 2H > install > 623H > j.H > rejump j.H (> j.22H > 6U whiff > j.H fuzzy)
Fuzzy setup that activates install in the middle. This isn't necessary, or even necessarily encouraged - you could just save your meter, and pop install if you get reset to neutral. But if you know you want to install this is a strong way to activate.
(starter) > 214H > 5L > 5H > 236H~4 > 623H > j.H > 2H (> install > j.6U > j.M fuzzy)
Alternate (easier) install setup which doesn't require your seal cooldown but costs juice directly and gives a weaker fuzzy position. Can be notably useful when you don't have the 100 meter until the end of the combo. Remember that j.M can be rolled, so you need to be ready to react to more options.
(starter) > 623L > 6U > 2M > 236H > dash forward > 214H > 623H > j.H > rejump j.H (> j.22H > j.6U whiff > j.H fuzzy)
The only midscreen install combo you need to know. Takes you corner-to-corner with only two dashes, AND it's a fuzzy setup. Need to delay the 623H dynamically based on distance to corner. Don't do more 6U > autocombo repetitions than you need to reach the corner - if you do too many, you'll get pushed out of range of autocombo and drop. Also, if you're too close to the corner, 236H will only hit twice and the combo will drop. At closer ranges, you can just do the standard meterless fuzzy that starts with 214H.
(starter) > 214H > 5L > 5H > 236H~4 > 623H > j.H > rejump j.H (> j.22H > j.6U whiff > j.H fuzzy)
Literally just the meterless fuzzy combo from the other page. If you were hoping for an install-specific route you might be disappointed, but this combo is great, and you shouldn't be spending your install juice any more than strictly necessary.
(starter) > 214H > 2M > 623M > j.2U > rejump j.L > j.1U > 5H > 236H~4
Install-exclusive tornado-in-front setup. Spends two install juice. Leaves you slightly minus, but not punishable. The minus frames don't really mean anything here compared to the j.214U setup honestly - either the opponent has a button they can press, or they don't. The 3 frames of difference will never matter. Basically, it's an identical setup, with all the pros and cons.
Meme combos with extremely varying degrees of practicality.
j.L > j.6U > j.M > repeat
Comedy loop you can perform on tall and semi-tall characters. Can do this on block too for a very long chain of fuzzy mix, though a savvy opponent can grab you out of it (why??).
(starter) > 214H > 2M > 623H > 5M > 236H > 5HHH (> 9U whiff > j.H fuzzy)
You were probably expecting something cooler. Spends one install juice for the combo, and a bonus charge if you want to go high. Not a very good use of your install charges and not particularly swaggy, so don't expect to get much use out of it.
(starter) > j.M > j.6U > j.M > rejump j.M > j.6U > j.M > 5H (> 9U whiff > j.H fuzzy)
Much cooler install-exclusive fuzzy route. Unfortunately it spends four juice just for the setup, does poverty damage, and to really rub salt in the wound the j.H fuzzy is manually timed. Effectively zero merit. But if you know it'll kill anyway, you can swag on them with this.
(starter) > 623M > j.3U > 2H > 623M > j.2U (whiff) > 2M > 623L (> 9U whiff > j.H fuzzy)
Another install-exclusive midscreen fuzzy. This one only spends THREE juice for the setup, and does slightly more damage! Again, zero merit. You could have just taken them to the corner instead. But isn't having zero merit kind of a merit in itself?
(starter) > 214H > 5L > 5H > 236H~4 > 623M > j.1U > f.L (> 6U whiff)
Refreshingly easy compared to the last couple of combos. Steals the corner in an attempt to catch your opponent not holding the block button. You would be shocked how often it gets blocked.
(starter) > 623M > j.3U > 2H > 236H~4 > 5M (> 6U whiff)
Spends two juice for a very reactable midscreen crossup. Your opponent is probably holding the guard button anyway. You could have just taken them to the corner instead.
(starter) > 623M > j.3U > 5H > 623M > j.3U (whiff) > crossup 2M > 623M > j.3U > 2H > 236H~8 (whiff) > 66M
Difficult install-exclusive midscreen overhead reset. All inputs need to be performed about as early as possible. Even if you hit the combo, the resulting conversion is extremely difficult to pick up. The one time I hit this combo in a match I got DPed out of it and cried.