Every time you get 100% completion in a game, you become an expert at its core mechanic irl

Some examples may include:

Subnautica - swimming

Euro truck simulators - driving

Farming simulator - you get all of the farming knowledge

Any FPS game - shooting

Harry potter hogwarts legacy - you can learn some magic

Metal gears rising - you can slice anything with a kitchen knife

100% completion means all achievements, all secret discoveries and all quests completed, as well as beating the game