You don't get much from pulling up on the pedal (though I'd argue that there are situations where you actually CAN pull up, like out-of-the-saddle low cadence climbing), but you can un-weight the pedal on the side that's coming up without your foot coming of the pedal and perhaps not returning to the optimum location. I see so many people with flat pedals putting the pedal spindle under the center of the foot, rather than the ball, and thus losing a lot of leverage. And they help with "push over the top, pull through the bottom" for optimizing your pedal stroke.