Because they are far more comfortable and luxurious than standard heads. Lock the door behind you and you have the entire lavvy to yourself, advantages include:
1. Plenty of room to stretch your legs as your ricker is strained to the very limit by a steaming great brown trout.
2. Grab handles either side of the chodbin, in case you need extra leverage.
3. You can even piss in the sink without anyone giving you a funny look.
4. There will be no ethnic gentleman asking if you would like to "freshen up for da punani".
5. You can have a thrap without fear of being disturbed.
6. Less likely to have seen immediate prior action, so you don't have to breathe in the fumes of someone elses dump.