Anyone know what this is for? It was in the bottom of the driver door.

 

That piece snaps in the lower door glass channel (the front channel below the rear-view mirror). Easiest to do through the speaker hole, window needs to be rolled up all the way. There should be a hole that keys into a peg in the glass channel. I have found that vibration or operation of the window pushes this piece down until it falls off the metal channel, it's not well secured from the factory. I ended up using very thin double-sided tape to keep mine in place, haven't had the problem since.
Cheers!
-Icer

 

Thank you! Do you know what the function of the part is? Is it to provide a padded surface for the bottom of the window glass? I ask because I already put my door back together and have to decide how important the piece is.

 

This piece helps keep the glass from rattling when rolled down, but more importantly, it will keep your glass from getting scratched. It also serves as an alignment guide for the regulator. 350Z doors do not have a window frame, there's also no rear glass guide/channel, just the rubber seal piece at the trailing edge of the door/window seal.

Me personally, I don't think Nissan would put this there if it wasn't important. But I think it's unlikely that your glass will be damaged if you don't cruise with the windows down all the time. Your call!
-Icer