Not very intuitive, is it?  We go to all the trouble to clean up contacts,
then rub insulating grease on them.  Until someone went to the trouble to
explain it to me, I had a hard time accepting the idea.
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I've always used dielectric grease on electrical contacts as a sealer to
keep moisture out.
New connectors are still subject to corrosion and white rust if they are
tinned.
With the dielectric grease they are sealed against the environmental
moisture and pollutants in the air. (Regular old general purpose will work
the same, used it on my boat trailer tail lights for the past 40 years to
seal the bulbs in the receptacles.)
I've removed turn, brake, stop light bulbs from Vanagons, Busses, Porsche
and other vehicles that were generously sealed with grease.
In these situations the receptacles appeared to remain in very good
condition.
With the higher heats of engine compartments the dielectric grease hold up
better against leakage.

Stan Wilder