The graph's author wishes to display vastly effects of vastly different scales on the same axis, without log scales comparing quantities such as 10 ^ 3 versus 10 ^ 9 becomes impractical from a visual perspective.
The graph's author wishes to display vastly effects of vastly different scales on the same axis. This is without log scales. This is comparing quantities such as 10 ^.3 versus 10 ^ 9 becomes impractical from a visual perspective 10 ^. Often the reason for the use of log scales is this.