In addition to the basebandy RF stuff that you expect the baseband to do, it also does real-time voice modulation and call quality things like echo and noise cancellation… things that high level OS would do too slowly.

In 2023 I suspect this is less and less necessary- apple silicon is very fast and a lot of voice comms goes through an app (FaceTime?) anyway but … I’m sure those capabilities are still in the baseband processor …