What would you use a physics object that doesnt collide with anything for? Wouldn't it just fall off the screen if it had gravity?
Particle effects. That's the major use. And a lot of particle effects don't use gravity.
Also, if there were no physics collisions, you could then test for sprite collisions, which would give you the ability to create detectors for your physics objects, or code your own collision routines instead of using the default physics collisions (which aren't conditionable).
For instance: Make a solid ball that's 32x32, then overlay it with an unsolid ball that's 34x34, give them the same mass, feed them the same velocities, etc. and you have a detector that will follow your ball without lagging behind.