If you’re extremely self reflective; nothing.

For most of us, a therapist can tell you a lot of things that you could observe about yourself but won’t admit to. Most people have a blind spot the size of themselves.

It's easy to underestimate just how deep the "humans are social creatures" thing goes. Your mind has capabilities that more or less only work in the context of an interaction. Communication orders your thoughts in a way that rumination doesn't. Cues from others calibrate your level of certainty.

The better question is why do you need a such an expensive, highly trained listener? It seems to me that some people (particularly women) get what amounts to therapy from very close and secure friendships. Universities also sometimes have programs to facilitate a similar kind of communication with lightly trained volunteers from your peer group. You may still get a decent proportion of the benefit from something like this. But it stands to reason that a pro is better at it.

Some things can be so intensively painful that we can't even acknowledge they exist within us, let alone begin to unpack and talk about them.

Speaking from my own experience it took me two years of therapy to get to acknowledge something very traumatic. Then the work could finally begin to start working on it.

Nothing.

You can learn ballroom dancing alone in a room too, it's just much easier with a partner.