Bring swelling down. After being on your feet all day, gravity pulls fluid in the only direction it knows... Down. You might notice them mildly swollen after a long day of walking. Keeping your feet propped up at the end of the day is relaxing and it can help return that accumulated fluid in your feet back to the rest of the body.
Depends. Should you? If you have "edema" or swelling in any part of the lower extremity, be it in the feet, ankles or legs then this is a good idea. Elevation will help aid speed up the return of blood and fluid and hence reduce the swelling. This reduction of edema will also feel better reducing any throbbing that may have been present from the overaccumulation of fluid.