Honestly, the short answer is because I think it's cool and because I can haha.

The long answer is because several features I appreciate would be either impossible or extremely painful to pull off on the stock OS. In no particular order, off the top of my head:

• I like the idea of having a fedora based OS that's stable but still as close to bleeding edge as it gets when it comes to the kernel, mesa and whatnot while retaining the steam niceties and getting easy rollbacks on top
• Easier to customize, I have my own fork with a couple of tweaks on top of mainline Bazzite
• Trying out new desktop environments comes as easy as rebasing to another image
• Btrfs with compression and deduplication on by default does wonders for space savings on proton prefixes
• It optionally installs Nix and it's my preferred package manager (and OS!)

For a more practical example on why I appreciate the more recent packages, I remember getting that new mesa release with considerably smaller shader caches months ago, I'm not even sure vanilla Steam OS already has it.

With all that said, it really does mostly boil down to my just feeling like tinkering a little anyway. There are cool advantages but they're pretty niche at the end of the day, I'm just the kind of nerd who loves experimenting. Hell, I'm considering test driving NixOS for the heck of it.