They die for any number of reasons and their shells litter the forests floors all over. In my first few years here I'd collect the shells and use them to decorate garden beds or whatever. It is actually illegal to possess to many of them, or to sell them, or transport them out of the state.
At some point in the last few years I ceased collecting them, but just let them lay. I have been tracking the above shell for the past year or so. It is where I take Bella for longer walks. At one point it was under a mossy log. Then a month or so later it was under a small cedar tree. That is where it has been for almost a year now. But when I check on it every so often it usually has collected a few more gnaw marks from squirrels or other rodents.
So these shells are good sources of calcium and probably other minerals for wildlife.
This past summer it appears to have collected a stick and now a few fall leaves. I am going to leave it as is and see what becomes of it later on.