The other answers are pretty nice, but I wanted to add something to them.
Garchomp is a pretty nice Pokémon in USM OU, because of a lot of reasons. With access to Swords Dance, a comfortable speed tier and cool coverage moves, Garchomp is one of the best wallbreakers in the meta. It is also a great Stealth Rock setter, as it can pressurise common Defoggers like Zapdos and Tornadus-T with a Z-Move. It's also a great tank, checking threatening Pokémon such as Hawlucha, Heatran, Scizor-Mega and Excadrill, weakening them heavily or chipping them with the combination of Rocky Helmet and Rough Skin. Even the tank set is a great Stealth Rocker, as it can pressure said Defoggers as well as other with Toxic, putting them on a death timer. You can also run Garchomp-Mega, which has added bulk and firepower, it's extremely difficult to check as its STAB moves deal a lot of damage to almost everything, and it also gets lots of opportunities to set Swords Dance too.
Garchomp's most common set is the offensive Stealth Rock setter one, and for good reason. As I mentioned before, it can threaten common Defoggers and set Rocks on their face, making it a premium choice for a lot of Bulky Offense and Hyper Offense teams. The set also runs Swords Dance, so breaking would-be checks like Clefable, Ferrothorn and Celesteela becomes easier after a boost. Garchomp forces a lot of switches, so you would get ample of chances to set Rocks, and clean mid- or late-game with Swords Dance too.
Then comes the tank set. Tank Garchomp is a nice choice on some Balance builds which struggle against Hawlucha and Heatran (plenty of teams have an issue with them), as it can take care of them very easily. As it can switch usually for free on Heatran, again you'll be getting lot of opportunities to set Rocks throughout the match. While it doesn't have the luxury to run a Z-Move, it's compensated bu Toxic, which is something any Defogger would have to be wary of, as nothing likes to be put on a death timer. Even without investment, Earthquake hits pretty hard, and Chomp has coverage moves such as Fire Blast which takes care of Scizor-Mega, Ferrothorn and Kartana. The speed tier also helps it a lot, as with maximum investment it outspeeds Medicham-Mega, Charizard-Mega-Y and Bisharp among others, and can heavily damage them or outright KO if they're weakened.
Now we come to its Mega evolution. Garchomp-Mega is a bit underwhelming, because of its lower speed tier, as it can get picked off easily by the aforementioned Medicham-Mega and Charizard-Mega-Y pretty easily. But still, it's pretty threatening with Swords Dance, and a mixed set with Stealth Rock is also a great hazard setter.
All in all, Garchomp has lot of uses in the USM OU metagame, and it has a lot of versatility too, so you can mould it in accordance to your team.
Hope it helps.