I have thought about this in embarrassing depth.

The first reason I can think of is when sex isn't an option. Maybe you're in a long distance relationship or something. It gives a good structure to what is essentially phone sex.

The second reason is as a form of foreplay. We already have sexy roleplay in the sheets to help set the mood, and now we're just using character sheets instead of bed sheets. Then your "what would you like to do" can be followed by a demonstration.

This is why I think sexy RPGs should be designed for couples.