Wasn’t it also for filling the gas tank static too?
Yeah, that could easily have been another reason for them. When cars starting having the filler neck at the back, instead of on the side there could have been some concern that static discharge during the fill up could ignite fumes emanating from the filler neck. Can't say for sure one way or the other; too long ago and way long before it got to the point where cars became interesting.
Nick