One reason for complying with a PPR requirement is in order to comply with Rule 40 of the Rules of the Air
40. An aircraft shall not taxi on the apron or the manoeuvring area of an aerodrome without the
permission of either—
(a) the person in charge of the aerodrome; or
(b) the air traffic control unit or aerodrome flight information service unit notified as being on watch at the aerodrome.
(b) does not necessarily mean you can get permission via the radio if an airfield is notified as PPR by telephone.
Last year the CAA prosecuted a pilot for landing at an airfield notified as PPR by telephone for allegedly failing to get the necessary telephone permission. Luckily he was found not guilty but was put to considerable time and trouble in having to fight his case.