In many places it isn't legal to ride a bike on a sidewalk. Sidewalks are for pedestrians. MUPs are for pedestrians, bicycles, and some other uses (skateboards and other non-motorized vehicles). However, many parts of many MUPs are nothing but glorified sidewalks and aren't good for riding on over 8-12 MPH. They cross too many driveways, provide worse traction than pavement (if they are simply converted sidewalks), and you constantly have to dodge pedestrians that don't understand things like staying to the right, that bike bells mean approaching danger, or that yelling on your left means they need to get out of the way.