It is a traditional way both for "breaking in" new women of the harem, and for disciplining the occasional uppity odalisque.
One of the Sultans I do business with likes to show off his harem. I saw among his bevy of beauties four or five naked ladies who were almost-perfectly tanned, but who had the most unusual tan-lines I think I have ever seen. The patches of gleaming white skin on them were in the shape of little arrows, three of them in fact: an arrow on each breast pointing to her nipple, and another just above her pubic area pointing to her crotch.
Ladies who are so-marked are the ones who were disciplined in exactly this way, i.e., by being staked out in the desert sun. Pieces of duct tape were cut out in the shape of the arrows so that the tan-lines described above would be formed on their bodies. These made identification of the ladies easier for the harem eunuch, who was charged with whipping them every evening after last prayers. This was an added punishment to that of being staked out, and the whippings would continue daily until the tan-lines were no longer visible.