1. To create an environment where the object under test can live.
2. To test that the object under test correctly modifies the environment.

Each use requires a slightly different API (#1 sytle APIs tend to be more flexible than style #2). I’ve used both styles of APIs in the past (and had been frustrated trying to use a style #2 API in a style #1 situation), but I hadn’t connected the differences with the reason for the differences before.