Android games are normally going to be inferior to Steam games, but this is still a nice option for games that aren’t available on Steam or can’t be played due to anti-cheat reasons.
You can read more about Waydroid here. There are reasons to believe valve is working on adding official Waydroid support to Deck, but until then you can install it this way.
Think of all the weird minigames that were only ever released on mobile; its Flash Games all over again! This is huge for emulation!