I assume some space reserved for swap files and caches (shaders, ...). Probably some place reserved for temporary download. Some place reserved for future update. Steam itself, the OS itself, maybe Java in case there's some java games/apps on the store i would be surprised if java is not installed, not sure how Proton works but i guess it involve some kind of windows files or image or something? I highly doubt Proton takes less than 1GB i think Wine takes 1.5GB minimum or something like that. I would not be surprised if Proton takes close to 2GB. From what i've seen it's quite good. Maybe Source is pre-installed. Y'all seem to forget it's 2021 and 10GB is like nothing nowadays .