Help me understand Sub-way Musicians.

Why would someone spend every day in a sub-way playing music? A sub-way is dark, noisy, and dirty. How much money can a sub-way musician make in a day. I can half way understand a Street Musician. Do you have to have a permit and how much does a permit cost? Can you run music business in a sub-way?

You street and sub-way musicians out there, tell me why you do it.