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.
It’s rare, but yes. There are some games with kernel anticheat that can only be installed on Deck with windows or android app support. There are games that you may not own on PC, or are much cheaper for the phone version. Some games get dramatically better battery performance if you run the phone version vs the PC version (the recent Roblox tool claims up to 4x the performance by running the mobile version). Finally there are some games that aren’t on PC at all.
