Arlo yeah the F-15 has them on all models. They are purely for emergencies. Especially if the brakes fail, Utility Hydraulic system failure, or a gear failure is probable. They want to get the bird stopped as quickly as possible. There is assemblies at both ends of the runway, take off or landings. They can catch the "barrier" as we call it if in the event they have to high speed abort or one of the above mentioned failures.
We had a bird divert to Khobar towers when I was in Qatar for a utility hydraulic system failure. On approach he came in to shallow and with his arresting hook down. It was trying for the landing barrier and not the run off barrier. But because of his approach his hook caught the first line instead. It yanked him out of mid air and slammed the jet to the runway. It collapsed the gear and did a bit of damage from one of the donuts on the line.