It might be better in another thread, but another concept might be "why perform WITH others". Have you ever performed with a group? As a for instance, have you played with an ensemble of other instruments?
In my case, I play with a trio, piano-bass-drums. The drums keep me honest about timing. The bass makes me pay attention to chord structures that may or may not be what I intend and adjusting on the fly is sometimes a requirement. And if I miss a note or notes, I HAVE to keep going, it's not optional. Also, it makes one concentrate in ways they wouldn't otherwise as it can lock you in to a timing you may not be comfortable with (too fast or even too slow), and also introduces a lot of background noise that can be distracting.
Might be a different subject, but it's what came to mind when I was reading the posts. It could be an ensemble. It could be a band. It could be another player either on the same set of piano keys or another player at another piano at the same time. It could be a vocalist singing with you. Lots of possible dynamics.