Every single “provider” that asks for a zip code is a scammer.  
They try to get as much info from you to later try to extort you for money. At least that’s my experience 100% of the time.  
I use a burner phone when contacting providers that don’t have reviews and online history  
This actually happened to me few days ago. Contacted the number and was asked weird questions about my location but I clearly stated that I’m looking for incall so my location was irrelevant.  
Next day I received a text on my burner number trying to extort money from me. Stating that he is the head of the brothel and I waisted his girls time and if I don’t pay I will be exposed and hurt by his gang.  
Don’t fall for it and if this happens to you just ignore it.  
That’s why it’s really a good idea to use a number that can’t be traced back to your personal information.

Scammers will place ads in several states and by your giving them your area code it clues them in on which ad you saw, so they can tailor the rest of the come-on for your location. Of course, it could be a legit provider advertising in several states simultaneously and not telling you that, but I consider that unethical behavior.

None have asked me. Seems unusual.

is that they are an agency posting numerous ads in markets all over the country and need your zip to coordinate an appointment with someone local to you. Obviously not the girl in the picture, LOL.

I have never heard of this, nor have I ever asked. But if I had to guess, I'd say it's likely asked because a provider that tours often wants to know what city you are inquiring about. Have you asked the provider? i'd imagine she could shed better light on it for you. xo

If someone needs an incall booking I will inquire what area, or zip code?

She was asking for your zipper code.

Do you have specific providers in mind? Are you trying to pay with a CC and their payment app needs it for verification (I sure hope not

That is perfectly understandable.

for financial depth based on the area he/she lives in.  Compton has a different zip code than Beverly Hills, but both are within LA County.    

The OP wrote, "I clearly stated that I’m looking for incall so my location was irrelevant." Furthermore, some of the posts, above, say that they are looking for zip code info for the location of the MEETING, not the residence. Large cities have MANY zip codes. The LA county area appears to span 90001 to 96161. That's a lot of zips!
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You can live in the upscale Riverdale section of NYC 10471 or the impoverished neighborhood of Hunts Point 10474. In either case, you might prefer to meet near the Empire State Building which is either neighborhood 10001 (34th St.) or you can use its own dedicated ESB-only zip code 10118 (5th Ave.). What are there, like more than 100 hotels within 5 minutes of the Empire State Building? If they can't process "10001" or "10118" then it's probably a scam.