Please don't post the Wikipedia explanation.
As far as I have learned, environmental contaminants can accumulate on the insulation surrounding the live conductors. This will distort the electric field potentially causing an insulation failure which causes current to travel along the outside of the bushing.
If the bushing is cylindrical this current is virtually un-dampened by the time it reaches the housing of the breaker, transformer etc. Whereas if the bushing is corrugated the current must travel along a much longer resistive path which lowers the current magnitude.
Can anyone verify/correct/elaborate on this?