From what I understand, many are forced into it, but many do it for fast money for drugs. My flatmate knew a lad who prostituted himself for drug money - he spent about 500 pound a day on it.
Incidentally, it seems that my neigbour slept with a prostitute - his girlfriend left the other night and was shouting allsorts at him, she said in her pakistani voice "don't f**k with pakistanis or we f**k with you back. you like your woman to be a bitch so now I'll be a bitch"