Different reasons. Most common I've encountered is that the person finds toribash too addictive and doesn't have the self control to stay away when they're in busy times like exams.
Generally they'll specific a length to be banned for and just come back when it expires but as I said, different people request it for different reasons. Some people just want a break and will request a perma and then post when they would like to be unbanned.
Most of the time we're happy to comply as long as you don't do it all the time or request to have all 30 of your alts banned too or are generally annoying about it.