does it work and what does it exactly do?

just curious

 

keeps the plastic rear window from rubbing on its self and from creasing-

i use a mat on the car side of the window (rear area below the window) and roll up an old thick detailers towel for the purpose that you are suggesting.
dont forget to get it when you put the top up!

 

so i wouldn't even need one since i have glass then right?

 

You would not need a "tube" or similiar device if you have glass. You may want to invest in the well mat to prevent scratche's on your rear glass however. I use a pool noodle and a Muz well mat for my 01' along with regular Plexus treatments and my plastic window is perfect.