Foam in cab

So, after gutting my 97 Extracab and rewiring everything (previous owner was egregious), I am in the process of putting everything back in its proper place.

My question: When I pulled out the speakers in the rear part of the cab, there was a general porous foam that was stuffed into place in the bottom (it was not molded or anything, just old). I replaced it with the stuff in the pictures, though not really knowing why. I figured it helped to keep water out of the door since there are drain holes in either side, or possibly it was for audio (doubtful)?

Should I have put foam in the bottom or left it empty? Feel free to tell me I am ill-informed.

 

Not sure why they stick that open cell foam in there. I pulled mine out & filled the cavities with closed cell foam to take up the airspace so there's less empty space to act like an echo chamber. Also used damping sheets on the large metal areas - they're some of the largest non-supported panels on my truck & probably some of the most resonant.

 

I actually recently took my truck to the audio shop to have them do some deadening in my little access doors…when I pulled the panel off they put a block of hard foam in my door kinda like that…I notice now the sound is less resonating as my door is very stiff…it sounds great though so I’m leaving it.