Plus you can play on your TV, mobile device, computer, etc, with the same game saves seamlessly moving between all of them.
No initial $400+ cost.
It offers a lot of the same benefits as consoles, including a monthly subscription that provides good, free games.
The downsides are latency and having to have an internet connection, and the latency is actually pretty decent. Oh, and having to buy the games from Stadia, even if you already own it on Steam, but that's no different than other "consoles".
It's a very compelling platform that they've completely failed to market correctly. With them giving up on first-party games, it seems pretty inevitable that they'll give up on the whole thing now, too. A lot of people never thought it'd last in the first place, and that's partly from their history and partly because Google didn't show enough commitment to it. Those studios that they just shut down were the best proof of their commitment.