I have no objection, but cannot understand why you have to call a provider twice. If you are a cop it would make no difference anyway. I know in some rare cases, a girl may have a window facing the corner you call from and is peeking out at you.

There are many reasons.  The one you mention is a valid one.  Others include;

A) the lady may have another appointment before yours and will not answer or call back until she is free

B) on the same note, to make sure you do not arrive early before she is ready

C) if you decide not to show up, the provider does not want you to have her exact address

Sinful is correct, she doesn't want to give you her exact address until the last moment. Shit happens. It's really no big deal, matter of fact, if a lady gives me her exact address early in our communications, I sometimes get concerned. The ladies call the shots, right up until you get comfortable, and even then they have final say in what happens. Good luck

Because I don't want a stranger I've never met knowing where I'm located if he has no intention of showing up. Not in some cases, but in most cases will she be peeking at you when you make that second call.

I have also used the two call in reverse when having an outcall. I wanted to know that the next knock on the door which would occur shortly would be the lady I was expecting.
Even with my ATF I always make or expect the last call to confirm I or she have arrived and we are ready for each other. Nothing draws more attention to your activity then knocking on a door and waiting around for an answer.

Ladies, please be ready with a nearby location...  some of us might need GPS if we're not familiar with the area.  

BTW, if you arrive early, don't sit in the lot...  looks suspicious and may draw the attention of security.  I once got the hairy eyeball from a patrol car...  the lady mentioned LE patrols on complaints of reported thefts from vehicles.  

If I'm early, I locate the call point & back track to the corner coffee shop & arrive back at the appointed time.  

You make the call & she guides your final approach.  I have had a lady say...  "I can not see you!"  Because I was at the wrong door.  I corrected my error as quickly as possible.

I have had a lady direct me around the block to park on the opposite side or in one case, in a nearby business lot.  

I've had a lady point me top the one leased spot for her clients (for her legit home business), parking in any other space caused problems with her landlord.

Once you are on scene, she can get you to her location with minimal fuss.  Directions through a hotel lobby, let you in a side door, or let you into her private space...  

It's well worth the extra minutes on my hobby phone!
skb

PS  Even my ATF, where I have walk in privledges (locking the door behind me as directed)...  I still call when I'm a few minutes out.