Mutexes lock code, TVars lock data.

If you lock a section of code (to protect data), there's no guarantee against mutations of that data from other sections of code.

If you lock the data itself, you can freely pass it around and anyone can operate on it concurrently (and reason about it as if it were single-threaded).

It's the same approach as a transactional database, where you share one gigantic bucket of mutable state with many callers, yet no-one has to put acquire/release/synchronise into their SQL statements.