I would think the two call system gives the provider a chance to "get ready" so to speak and make sure you are there at the right time to avoid meeting someone else leaving or the like.  if you are the first of the day, it is also nice to have the woman completely ready (dried hair, etc.)

With some of the women I have seen, I have arrived very early, and the two call system prevented me from arriving to her place too early.  Traffic in Boston can sometimes be a bitch, but sometimes it moves quite quickly.

Some women also can see the place they tell you to go to, so they can "scope you out" before they see you in person without you knowing they are looking.

I have had some women "talk me in" without revealing their address at all.  They merely watch from their window while I am on a cell phone getting step by step directions to their location.

For a woman working in a hotel, the two call system can also help when they are checking in right before you.  The cannot possible know the room # before they check in.