NOTAMs distribute aeronautical info in a systematic way so you don't have to call the airport.
Not always used by UK unlicensed airfields.
Talking on the phone before setting out allows a better briefing, including any topical information, without tying up a radio channel explaining noise sensitive areas or whatever. Maybe they want to point you to a web site to see a map. Bit late if you call when you're in the air.
Like Fuji, I do sometimes decide in mid-air to go somewhere, but always ask "Is PPR on the radio OK?" or some such, and be prepared to go away if not.
The bottom line: airfields are generally private property, and have the right to require PPR by phone. They may not mind if you come along without PPR, but why take the risk if you know before you set out where you're going?