I'm going to combine all of your questions and say it's entirely contextual. I know I am very comfortable/confident with traveling, so I rarely "need" it. I will book it for myself only if I have a tight connection. If I am booking something for a family member, I'm more prone to booking a greeter service as I know it will greatly help them and ease any stress. I am a travel agent so I am able to reach out to contacts directly.

As other's mentioned, you usually do not need this if you're flying in First class, and sometimes it's included with a hotel booking. It's certainly the most useful where immigration lines can be a zoo (JFK/EWR, MIA, LAX, LHR, DXB, HKG, etc.). It's also nice at enormous airports like IST where you can be driven the entire way.

I'll add that not all VIP greeter services are created equally, and some are MUCH better than others. To take it further, even the good services have specific greeters we will not use because of negative feedback. I think people would also be surprised to see that not all are very expensive and can get you through the arrivals/departures process seamlessly for a small amount of $$.