I think sometimes they're nervous about being recognised so they want to make sure we're not near a family home or workplace. Other clients want to know if we're in a house or hotel because they have preferences either way.
Other times it could be police, or pimps keeping themselves informed about what's happening in their area.
Of course some are just weird but I think nerves plays a larger part in clients irritating behaviour than we sometimes give them credit for.
And others think nothing of us except for demanding what they want as quickly and easily as possible. Those lot can sod off.