Isn’t performing for ‘exposure’ necessary?

Yes, but good exposure is incredibly rare, and empty promises are everywhere.  It's important to know the difference.

• ‘good exposure’ means you are guaranteed a large number of people who are likely to be interested in your music.  
• The term is so commonly mis- and over-used that booking agents will try to sell musicians on the great exposure opportunity, then mention that there’s no built-in draw, without even realizing they’ve fully contradicted themselves.
• Good exposure does not mean you shouldn’t be paid. Most of the best exposure gigs pay well.  
• Busking can be very valuable for exposure, promotion, and practice, and performers get 100% of the proceeds.