The well-known CLAWS5 tagset, for tagging English language lexemes etc, has one tag
PRF, specifically for tagging the preposition "of", while other prepositions get the tag
PRP, which is for all prepositions except "of".
Apparently, this kind of thing has some use, since two revisions later, CLAWS7 has separate tags also for "with/without" and "for".
What is the motivation behind these distinctions?