I don’t know, but my cat does it, too. Usually my husband’s – probably because he never moves at night. Sometimes mine, though she prefers under the cover behind my knees. If our daughter is in our bed, she is always on her feet. When she is feeling brave, she goes to daughter’s bed and sleeps on them.
I think it must be warmth or affection because she will go to other parts of the body, but feet seem to be her preference.