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.
I’m convinced that is about bringing Oculus/Meta Quest VR games to the next Valve VR headset which is rumored to be stand alone and run SteamOS.