New York City is home to thousands of musicians (of varying levels of talent and professional stature) struggling to make a living. There is a complex code of hierarchy and honor governing street musicians that play in public spaces.
In the park during the Summer, anybody can set up a portable microphone, grab a guitar and have a willing (or unwilling) audience. Bands will often promote club jobs (also often non-paying) with these impromptu performances in the park. And some musicians just come to the park to practice.
The park is also home to more formal performances with a summer stage on the Rumsey Playfield and occasional large concerts on the Great Lawn.