What Use Is An F-call?

What use is an F-call? As you gain more experience as an Amateur I've found that I spend more time listening and less time talking. I set my radio up in my car, get comfortable with a clipboard and headphones and tune up and down the dial listening for stations. I do this regularly. I've listened to the bands enough to know that each band has its own "feel", the crackle and hiss of each one slightly but distinctly different. On occasion I've had the fortune to be able to spend more than a few hours listening, scanning and attempting to add to my QRP or low power contact list. On these rare but rewarding outings I've observed something that you can only really get by experiencing it. Quite suddenly, within the space of 15 seconds or so, the band changes. The closest analogy I can come up with is to think of an out of focus camera that suddenly shows the picture in full and clear detail. It's not the same as what happens on air, but it's the best way I can describe it. When you listen to a band, y