GeeTR is right about everything he says... LoL
Your talking about heat sink compound and igniters or transistor unit; a solid state, high amperage switching device. As such, it produces an amount of heat, and will complain if it cant get rid of it. Instead of OEM sinking it to cam valley cover (which IS metal but ISN'T cool) to help with heat, people suspend them on longer threaded bolts, or bolt to the firewall to move stop heat going the other way, from cam cover -> igniter.
We are talking about the coilpacks, which aren't as sensitive to heat, but to a – GROUND. Having that good ground is paramount for a decent electrical field creation and collapse.
Even new leads will get dirty, as I'm not talking about a few mm of untreated lead post, but .001 of a mm.
To clarify…
To prevent corrosion people use almost any petroleum based grease on the terminals, much like filing the surface of galvanized steel before welding, and then spray with rust inhibitor (or just grey paint) after you've welded it… to stop that surface oxidization.
There's enough contact from post to lead clamp that conductivity isn't effected too much.
+1 on the above, is actually being able to find some of the grease that's specifically electrically conductive (in addition to it being a simple barrier to atmospheric oxygen)