As Odai said:
"As far as I understand it's just there to protect traffic on/around the aerodrome."
Basically, the airfield operator gets some say on who flies in the vicinity of their airfield. They might allow people in non-radio but PPR, they might say people have to have radio and PPR, they might just let people call up on the radio.
They might even, like Oban, say PPR by telephone only, and then try to turn away a pal of mine who called up on radio. He then phoned a minute later on his mobile interfaced with the intercom!!! Ha! Ha!
It also gives a little bit of conspicuity on the charts! Otherwise, Farmer Giles with a 40 hour a year Piper Cub gets a small circle on the chart. A busy microlight training airfield might also have the same annotation - see East Fortune or Strathaven. Or a busy GA field, see Fife.