Its all about deterrence and control of airspace. Enemies are not stupid, if they know that F-15s are in the air then they are very likely to stay the hell away

Its all about deterrence and control of airspace. Enemies are not stupid, if they know that F-15s are in the air then they are very likely to stay the hell away. No combat encounter, but mission success and other assets protected. If the F-15s weren't there then the enemy aircraft would have been free to wreak havoc.