At all airports I have been in, thre have been many types of non-check or boarding in desk I have seen so for example airline services, car hire, currency exchange or even airport bus/taxi
Why it should be implemented:
This could be implemented quite easily in my view since you could add a vairiable such as a 10% chance on arrival of going to a car hire desk or a 15% chance during departure of currency exchange. Possibly when desks make money the airport could take a cut so 5% of currency exchange.
It could be implmented in a minor bug fix by adding a drop down menu to info desks like airlines to stands. I have decided agaisnst adding each type to UI since this would probbably add much more dev time for you. Also with regard to extra airline desks maybe they could change ratings.
For car rentals it could easily just be a new zone. Requirements: X-Infodesks/cashierdesks (or both?) + Y-Parkinglots ( Maybe Z - Beutification elements like plants or something). Car-rental zone and parkinglot connects the same way stands vs boarding desks or Baggage bay vs Baggage claim. Same mechanics as for shops, You build the zone → fulfill requirements → Sign contracts → get rent → gets bonus if you meet sales