Waxed paper (??) between baseboards/door casing and sheetrock?

I've removed some baseboards and door casings and found that most of them have what looks and feels something like waxed (or perhaps plastic-impregnated) paper between the wood and the sheetrock (which has a textured finish, if that makes a difference).
What was the purpose of this, and do I need to install something similar when I reinstall the baseboards and casings?
Perce
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used by painter when originally finished no need to use again

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That's a head scratcher. Any evidence that the paper was cut in place, like a thin knife mark where it meets the wall? Only thing I can come up with was that the trim was finished in place, and they wanted to avoid mucking up the wall, so they installed it over paper, then trimmed afterward. Or maybe that they knew the walls would be textured later, and installed the trim wrapped to protect it from that. Sometimes you have to get inventive to schedule all the trades in there when you can get them.
As a kid on construction sites many years ago, I do recall some extra-fancy hardwoods being delivered wrapped in kraft paper, but never saw any of it installed with the paper still on it.
aem sends...
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I would say the walls were painted before the trim installed, then it was installed with wax paper hanging out from behind to protect the walls while finishing. Done it myself at times.

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