The black plastic thing is falling off, is it needed for anything?

Hi Andrei,

I wasn't able to find any information about what that piece is, but it's part of a larger piece that covers the underside of the midframe. If I had to guess, I'd say it's got to do with suppressing electromagnetic noise on that large flex cable that runs beneath it, and/or providing heat or shock insulation.

Given that phone makers are extremely cost conscious I assume there's a purpose for it to stick out that far from under the midframe and that it's most likely related to that flex cable. It doesn't appear to have any electrical characteristics that I can see, so if it was my phone I would probably take a piece of electrical tape and just tape it back in place. Chances are your phone will still work fine without it, but without knowing what it's for, I'd say it's best to leave it in place if at all possible.