In the "Good Old Days" they were there to escort the aircraft and intimidate the HiJackers.
Today; er, The fighter is there to do all the above. And To Shoot It Down Befor It Can Be Crashed Into Someones (like you or me) Local Town Causing Mass Death And Distruction.
There IS A WAR going on out there!
If its got the BAD guys in it then = SHOOT IT DOWN.