Comment by gassi

Ideally you would use both, and the community would build integrations that allow, say, EmuDeck to pull games from the server onto your Steam Deck and sync saves back to the server.

I usually play on Windows with Playnite (https://playnite.link/), and the plugin (https://github.com/rommapp/playnite-plugin) allows Playnite to pull and display the list of games for each system, after which I can install the ones I want to play onto my PC.