nonSteam games, pirated games, maybe your setup or you yourself needs different settings for whatever reason, console emulation, and more
from the above link:
It is based on Valve’s version of WINE (Proton) and its modifications (Proton GE). It includes a set of scripts combined with wine-proton itself, a Steam Runtime Sniper container with the addition of ported versions of MANGOHUD (displaying useful information in the game window: FPS, FrameTime, CPU, GPU, etc.) and vkBasalt (improving graphics in games, very good in conjunction with FSR, DLSS) + many already configured optimizations for maximum performance.
One-click automatic installation (on the AUTO-install tab) of popular launchers such as: WGC, Epic Games, BattleNET, Origin, EVE Online, RockStar, Ubisoft connect, League of Legends and many others has been implemented.
For console game lovers, there are many console emulators available (under the EMULATORS tab): PPSSPP, Citra, Cemu, ePSXe, MAME and many more…