There are a few models.

- They have majority of popular games already installed but you need to log in to your own steam account and only have access to the games you already own.

- They provide an in-house account and offer games available in that account to be played. This is usually for cafes offering playtime for console games.

- Partnerships with games studios/publishers, but really I only know about this in Korean PC Bangs for specific games where some content is locked. For example in League of Legends, you unlock playable characters over the course of playing the game so a low level account might not have the full roster unlocked. If you log in your account to affiliated PC Bangs, you have all characters unlocked while playing there.