It's a lot more than just Steam, it's a custom kernel, custom CPU scheduler, additional drivers for game controllers, drivers for handheld devices and a bunch of other tweaks and tools (Bazaar, Lutris, MangoHUD, ujust scripts etc). But more than all that, the biggest draw for Bazzite is that it's an immutable distro with atomic updates, so updates "just work" and it's very very hard to break the system.
And in the rare event you get a bad update, you can just boot to the previous two images right from the boot menu, no need for any commands or restores - just boot the image and keep using it without any worries. You can pin known good images too, so you know for sure you always have a working image you can boot into. And you have access to the previous 90 days of images (via Github), so you can switch to any old image (or the latest beta) for bug/regression testing, without needing to do lengthy backups and restores.
All this makes it ideal for someone who just wants their system to work without worrying about updates and stuff, getting you a console-like experience on PC.