Why would women use the chasity belt to enslave a men?

To metaphorically castrate him, as all women seem to like to do.

Just (half) kidding

you must deserve it or they wouldn't do it to you.

you have been a bad, bad boy i assume.

As part of a BDSM relationship, to assert dominance and/or humiliate their partner.

To keep him from cheating. Also, some folks are into S&M, and the guy actually likes it.