Because:-
Air-to-Air refueling means.
Increased range (distance) and endurance (Time).
PAF has to increase its endurance (on station time on one particular point in airspace) with minimum number of aircraft so as to keep those points plugged for maximum time with least number of fighters. So, with AAR a fighter can stay in air twice as much the time than it does without in- flight refueling. AAR is carried out under the cover of Air Borne Early Warning.
Hence, AAR and AEWC makes any airforce the most potent force against which one has to think 10 times to launch an attack.
Hope, I made you understand.