Since the Steam Deck is a portable you might want to bring your various media with you from Music to Movies — it supports a huge amount. Also manage Photos, Games, TV Shows and more. The plugin system is great too, there's lots you can add into it. The idea is to give you a good looking way to manage and view it all.
Giving it a short test today, it seems to work just fine in Desktop Mode as it does in Gaming Mode when you add it to Steam.
To install it on a Steam Deck, simply load up Discover and search for it. Nice and simple. You can see various guides on the GamingOnLinux YouTube.