1. It's often uncommissioned = most interesting/unconventional
2. It's often politically or socially conscious
3. It beautifies public spaces and makes them more interesting
4. You don't have to pay to enjoy it (and many street artists actually have the art school training)
5. It's more available to the public than art in a gallery or museum (anyone can enjoy)
6. p.s. WHY NOT?
Sometimes street art is left alone by the city, or actually protected, as is the famous "Crack is Wack" mural, which I visited in NYC a few months ago. Other times, it is covered up.
Here are some great blogs and sites if you want to see some street art, stencils, stickers, etc. I really love the "Eco" street art!