It notifies the Provider of your impending arrival and allows her to prepare for and observe your arrival to satisfy her concern that LE may be close at hand.

She may not be ready once you arrive.  

Also, she may still have another client with her.  Trust me, you do not want to arrive at her door the same time her previous client is leaving.

She gives you a call point.  If you no show or call the coppers...  you still do not know her exact location.  In the old days, a gal would be able to observe the call point & it still happens sometimes.  You can not assume she will not observe.  

On one occasion, I had a date with a gal I'd seen several times...  but in a different location.  I arrived early, confirmed the location but withdrew to wait at the corner coffee shop.  (Sitting in a hotel lot IS conspicuous!)  So I called her as I turned into the lot & she (apparently glancing from her window) replied "I don't see you!"  I told her I was pilling up...  but she WAS observing.   I'd say about 50% of my first meetings, I was observed prior to entry.
A young MILF gave me an address.  I went to that location & that number didn't exist...  She came up behind me from across the street.
I call at the call point & the gal stayed on the phone with me through a series of turns.  I ended up at the same place, on the other side of the street, in front of her door...  and she was sure I wasn't followed.
A stunning lady had me call from a major intersection...  gave me directions, a couple of blocks down & up a side street.  She told me when to stop.  She clearly observed me arrive.
A lady gave me directions, watched me arrive from her window & directed me to her stairs.  

I do not mind.  It's a small step at security.  It's how the game is played.

instruct you to call when you arrive. Then she will most likely give you turn by turn directions to get to her place. It is all about following the lady's protocol for her safety.

Sometimes to get a visual on you.  Other times to keep location a bit more secret.

Of course you always have the other type 2 call system....
1st call sets up bust,  2nd call gets lawyer involved.