a) any corruption it won't take AGES to start
b) converts old part styles from previous emules, mldonkey, edonkey, etc to the new version
c) reindexes and adds to the downloads.txt, history.
vs.
a) cheese it
With me it didn't take any time at all, and didn't lag out emule, whereas when emule used to pick up an entire temp folder it would freeze and be unusable until it had fully re-hashed and spat out any errors, which with the number of part files i had took hours