What Use Is An F-call?

What use is an F-call? Recently I had to repair one of my antennas. It had fallen on hard-times, strictly speaking, it fell on hard concrete when it fell off my fence, but the outcome was the same, it broke. When I had a chance to see inside the broken component I realised that it was plastic. In my case the part that broke was the Phase Coil, it's the bead in the middle of my multi-band 2m/70cm antenna. The part was made of two metal grommets, with a plastic pipe between them. Inside is a copper coil - presumably tuned for purpose. When I saw inside this coil I realised how fragile this part really was. Until then I'd had no idea that the part was made of plastic, let alone that it might break so easily. I've said before that I've set-up my kit at least every week since I purchased it, so at least 50 times or so. It's not sitting on a desk, bolted in place, set-up just so, it lives inside a rolling toolbox that I take from site to site and then it gets taken out and put together each time. I started wond