I have not followed threads on this topic, but there are four possible explanations, three of which are more palatable than the other.
A good explanation: He's cautious about his own safety. He's trying to get a vibe on who this person really is and whether he should invite the client into his home.
A good explanation: He has a day job and he has to maintain some level of discretion and wants to make sure the client isn't his boss or the guy who is in the next cubicle.
A good explanation: He is running a business and wants to make sure the guy is a real person who is actually going to show up and pay help him pay the rent.
A bad explanation: He wants the client to be hot.