The plastic sounds like a wrap for the paper. I have a number of those, got one with each spindle, they keep the paper from unwinding.
I also got some black plastic pieces - not the pieces for the 3" paper, curved like a ramp, about 3" long - not sure what those are for.
I *assume* that the spindle has the extra tension built in? Never bought one, my vendor told me not to bother.
Not sure if they help or are needed. I have heard that paper feed can cause micro-banding, so they may help.