I replaced a set of Spark Plug Leads on a GU Patrol 4.5L Petrol yesterday
I just did it as a matter of course while changing the spark plugs and doing the filters.

The plugs i removed where in near perfect condition and leads in perfect condition
Bright shiny ends where they plug into the distributor and connect to the spark plug, nice flexible silicon leads, no marks on them, nothing

I replaced them a some 8mm silicon leads, which subsequently were too fat to fit in the stock lead holders, making this a bit messy.

I know they say to change leads after a certain amount of km's...... But why?
Does the actual cable break down?? Or is it the terminations begin to become high resistance or something?

Engine ran perfectly smooth and quite before, same after.....

So, what is it with leads needing to be replaced at a certain interval???