Those crime bosses eventually want to buy a new mansion, Porche, a yacht, jewellery/stuff for their wife/gf, you know, anything to actually enjoy their life in real world. They eventually need to exchange their ill-gotten gains for real money. They can't stay in BTC forever. Basically they want to turn the BTC they got from their criminal activities to a clean cash not linked to crime. There are various way they might go about it, basically similar techniques as money launderers use.