There is a thin layer of sponge-like material lining the back of the tray-end (the part that comes into contact with the front bezel when the tray closes.
I believe it is simply there to dampen the "clunk" you inevitably get when these two hit each other.
Afreey CDROMs and the new Artec 52x writer also have this "feature". I personally think it's a cheap way of enabling the factories to produce a tray mechanism that doesn't need to be so accurate when closing and getting the two surfaces to the correct placing. Wouldn't worry about it either way -- no harm in it being there.