Sheldon (as always) has something interesting to say about that. With clips and straps you can place your foot at many different points. You can push with your toes or your mid-foot or anything between. This basically changes your seat height on the fly. When you can't change gears or stop pedaling this can be sort of handy.
I tend to push my feet in as I go up hills and drag them out a bit when I spin down hills.