What Use Is An F-call?

What use is an F-call? When you get an amateur radio license, you learn that different license classes have different power limits. The basic power limit in Australia, the foundation license, or f-call, has 10 Watts as the limit. The standard license has 100 Watts and the advanced license has a maximum legal limit of 400 Watts. It's natural to think that more power gives you more reach, but realistically, what does that look like, what is the difference between 10 Watts and 100 Watts? Can you really notice a difference? From my own experiments, I can confirm that it's possible to talk to the opposite side of the earth with 5 Watts, but was that a fluke, or is there more to it? What is the difference? All things being equal, that is, the same radio, the same conditions, the same antenna, the same location, etc. - the difference between 10 Watts and 100 Watts is a 10-fold increase, or, if you have 400 Watts at your finger-tips, that's 40 times more - right? Not quite. If you recall, a dBm is a decibel-mil