Ripped a thin black layer of plastic off the back side of the screen

Just received the replacement kit for my iPhone 6s battery so I opened the iPhone and I realised that I just ripped apart this thin plastic layer on the backside of the screen. There is some kind of oily stuff underneath that layer. It looks like one end of that black layer sticked to the battery adhesive strips I installed the last time I replaced the battery; pulling it apart while I separated the screen.

Should I be worried? What is it good for anyway? Should I continue the battery replacement? Would it be dumb to rip the rest off and clean the oily stuff? Though I suppose there is a reason it exists…