Because it is useful in comparing rates.
Here's a nice write up: http://www.health.state.pa.us/hpa/stats/techassist/arithlog.htm http://www.health.state.pa.us/hpa/stats/techassist/arithlog....
I know what a log chart is and how to make one, it's just that I fail to see how making either a time axis or a per-mille axis logarithmic will clarify things.