Thin plastic gasket

Sis-in-law broke a (manky plastic) light switch and bought a nice shiny chrome "flush" switch as a replacement. Connecting the thing up (complete with earth which wasn't there previously) wasn't a problem but after screwing the thing to the wall I found a thin, clear plastic gasket in the packaging. The instructions say this should be fitted between the faceplate and the wall... but why? There's a glued-on foam gasket on the back of the faceplate anyway, so what is the plastic one for?
Any ideas?
Hwyl!
M.
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Martin Angove: http://www.tridwr.demon.co.uk /
Don't fight technology, live with it: http://www.livtech.co.uk /
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When you clean, or even just touching the face plate to use the switch, you can touch the wall around the sides, so the thin plastic film is there to stop marks getting on the wall. It may actually stop any tarnishing from the metal plate marking the wall on some makes of switch gear.
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BigWallop wrote:

Oh. Is that what its for. I binned all of mine.

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I only fit them for customers. I binned all mine as well. :-))
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