I’ve got an old beater that I’m driving around and I needed to take out the headliner. I don’t want to put much money or time into this project, because the car is simply not worth it.
I removed the shell when I took out the headliner, and it would be cool with me just to leave the whole thing out of the vehicle.
My question is whether or not the shell serves a purpose that would make it worth my while to keep it around. Does it provide sound or weather insulation of any sorts?
I don’t know where you are at, but here in southern California, the sun can heat the exposed metal surfaces up pretty fierce. I imagine that touching the inside of a bare metal roof may lead to some painful owwies.
I always imagined it was to both protect the occupants from the hot roof and to act as a bit of insulation for the car. There’s always the aesthetic value of it too. 
It helps reduce “road noise” inside the passenger area, too.
Dang, beaten to the first 2 answers I thought of. Here’s number 3:
[lolcats]
Itz in ur car,
keepin ur head from bangin
goin over speedzbumpz
[/lolcats]
Lots of old cars didn’t have these at all. The headliner was just some nice fabric suspended from some wire bows that crossed the underside of the roof. Lots of vans still don’t have headliners at all in the back. So, there’s lots of precedent for it, but you lose the advantages pointed out above.
I’m confused, I know what a headliner, but what is the ‘shell’ you mentioned? It seems like people are using the terms interchangably, but when the OP said “I removed the shell when I took out the headliner” it seems like they’re talking about two distinct things? Is the shell just the trim around the edge, or something else?
Beats me, I have no clue what the OP is talking about.
The headliner is just the fabric that covers the shell. You can easily peel this off of the shell. The shell is about 1/4 in. thick and has some sort of insulating material to it. The material used might be different in newer cars, but the car in question is almost 20 years old.
Well, no. The headliner is the totality of what lies under the roof. It may be just fabric, or just hardboard, or it may be a fabric-covered shell. In your case, the fabric and shell together comprise the headliner.
In which case, it probably serves to keep the heat in or out of the car (depending on the season) and reduce echoing in the cab. I wouldn’t worry about it. I’ve driven plenty of vans with no headliner whatsover, just metal and had no problems.
They probably also serve to make it look nicer. The ‘shell’ will give the installer a smooth surface to put the headliner fabric on.