In logarithmic scale you can see clearly small error rates.
In linear scale, if the Y axis spans from 0 to 1, you could not distinguish for example 1e-3 from 1e-4 or 1e-9 or less.
There is a big big difference in performance between those values. You could not distinguish a good system and a bad one with a sight. In log scale you can.
Regards
To give a better visualization of the fine error rates that have different order of magnitude but are all very small and looks close to each other in linear scale.