There are three reason people ask this.
1. Make sure you are not pre-recorded.
2. You have no pictures on your profile for someone to get a sense of your body shape
3. They are trying to get you to something for free.
The first two are important to a customer making a decision about taking your private. The problem is the third.
Before i take a model private I usually ask her " How much for standing up and turning round ? "
Main reason I like to see the whole body and the way she moves.
Second reason I get a feeling if she charges a lot or little for this, gives me an indication regarding other requests I might have..