Lumia 640 XL back cover

[randals]

This might seem like a dumb question, and, it may have already been posed here somewhere but, I can't find it. There are a couple black patches on the inside of the back/battery cover of my 640 XL. I've bought some back covers that have them and some that don't... Are they essential? What purpose do those serve?

 

[Scienceguy Labs]

I'm thinking that the black pads take up space and add a little pressure against the battery to keep it from becoming loose.

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