Anytime you install a new game on Steam, it will also install all necessary prerequisite runtimes beforehand. This means that each game is holding onto a handful of installation files that may not take up much space on their own, but if you have 20 or more games installed, that space can add up quickly. Steam Cleaner is a utility that looks for those redundant files (in GoG, Origin, and Uplay too) and removes them if they’re not vital.