What is the rationale behind placing arrestor gear on a long runway?
I can think of 4 reasons:

Pilot Training to use the cable arresting system
Rejected takeoff, fighters don't usually have reversers and may take long space to arrest
Under certain bad runway conditions, like ice, the required landing distance may be very high, and a military airport should work (ideally) under any weather condition
Runway partial unavailability, if part of the runway is unavailable, with an arresting cable system you can still use the other part. This is especially important during a war, when the enemy may damage half of the runway and you don't have time to fix it. Military airports are designed considering also these possibilities.

Source for 3 and 4: https://www.wbdg.org/FFC/AF/AFTTP/afttp_3_32.12.pdf