They are baffles, help deaden the sound from inside the door and keep speakers free from any water infiltration. Also helpful as a gasket, so no vibration from the speakers. I would only use these in doors, not for deck speakers as you lose some bass response.
Good protection to moisture
from Pittsburgh, PA
A very good addition to protecting new products from potential moisture issues shared by many GM truck owners. Used these as "insurance" for the new speakers. So far so good. Nice stock fit as well.
Nullified Bass Response
I too thought these items would provide some protection from the elements, however when I installed them the sound was so muffled and isolated (to me) it rendered the speakers useless. As soon as I removed them the sound I was expecting was there. The OEM speakers did not have anything like this and did not appear water damaged in any way. It appears I did not need them at all.
While I suppose if you must have a very specific moisture issue them they might be useful but I regret ordering them. By the way, I ordered these because they popped up as (what I assume was) recommended by Crutchfield. I am disappointed. I feel like I just wasted my money. I have not use for them now.
from Bellflower, CA
It makes sense to have baffles to direct the sound more accurately. These did a good job. They were easy to poke holes in for the mounting screws.
from Grinnell, IA
These work spectacular for protection and sound quality