Is cloud gaming required to buy a game?

Two separate things

Buying a key gets the game activated on its native platform (Steam, Ubisoft Connect, and so on) — that part works exactly like buying from any other key reseller. Boosteroid Cloud is a separate streaming service that lets you play an owned title instantly in a browser or on a TV, without installing it.

When it's worth using

• Your device can't run the game locally
• You want to try a game before committing to a full download
• You're switching devices and don't want to reinstall

You can always install the game normally instead — cloud streaming never replaces that option, it just sits alongside it.