Take a look at this explanation: http://www.fool.com/foolfaq/foolfaqcharts.htm

Ah, that makes perfect sense. With a logarithmic chart, each change is seen in the context of the current value. Each change is not equal to every other change. When bitcoin is worth $1, and it goes up one more $1, that's a much bigger change than when at $300 it goes to $301 but to have the line the same length is kind of misleading.

I think, because money as any other standard is always a network effect. Networks grow exponentially, in a self-reinforcing manner. Or they don't grow at all (when at a point of saturation).