Above all else, the answer is that it is FUN. Here are a few other motivations:

• Opportunities for emerging artists to show their work, build community among different artists/types of artists
• Access to the arts for everyone, with no costs or stigma about going into museums
• Participation, building an expectation that people should not be spectators, should not wait around to be entertained
• Build community among urban residents
• A place for young families to go, and opportunities for children
• New uses for underutilized public (or private) space (depending on your city and circumstances)
• Create a culture free from branding, commercialism, buying, selling… show people that these things are not essential for a good time
• Demonstrate and celebrate the abundance of creativity, passion, community – which are all free and abundant and unaffected by economics