Bazzite fills a SteamOS-sized hole with a decent level of hardware support. Unclear how long that'll be the case - I don't see it surviving the release of a GA SteamOS.
Bazzite also has a much more frequent release cadence which is important for the kernel and Mesa. SteamOS only ships a major version every year.
SteamOS 3.7 is still on Kernel 6.11 and KDE Plasma 6.2, for example. Bazzite is 6.17 and Plasma 6.5.
This matters if you're using more recent hardware or want the latest driver optimizations. My 9070 XT is supported by Bazzite, SteamOS won't even boot.
SteamOS release notes are public at https://www.steamdeck.com/en/news, it still uses a 6.11 kernel from September 2024.