I think the answer is that the reasons for PPR vary.
It may be a private field, in which case the owner is simply entitled to require PPR absolutely. It's his private property. Most of them are in any case operating without a planning permission to have to keep it under the 28 day rule, which means keeping an ultra low profile for 10 years minimum.
It may be a very busy airfield which has serious parking issues. If 10 planes all turn up they may have nowhere to park.
Or it could be like what they have in Bournemouth which is a man in a portacabin who has got a concession from the airport to pick up landing fees, and to hand out PPR numbers. He also likes to tell people that they have to go via him even if going to one of the based maintenance companies, etc. ATC there rarely if ever ask for the PPR number.
Across Europe, many airports are PPR but many are not. Generally the big H24 ones (e.g. Prague) are not PPR. The UK is unusual in that Gatwick etc are PPR but that is because the "management" there has decided to push out GA totally, by giving the handling conecssion to Harrods and Signature who both charge the same rates - about £400
You should
always contact an airport before flying there. Saves all kinds of hassle. Ask about opening hours, fuel, prices, etc.