The reason why some people collect old mattresses is to recycle the materials and reuse the materials in the mattresses.
Mattresses contain materials that can be used as insulation such as in dog houses and the springs in the mattresses can also be recycled for metal.
Some people also use the old mattress materials as bedding for dogs and other pets.
Box springs also contain wood which can also be reused in making things including dog houses.
I collected a few old mattresses off the curb before and used the material as insulation for a dog house.
Then I used the wood from the box spring or bottom part of the mattress to make the dog house for my dog.
The foam, steel coils and fibers from a mattress can be reused and repurposed and recycled into new products.
Many people just simply throw the mattresses and box springs out when they no longer need them which is okay as well if you don't have any way to recycle them.
A prison close to where I live also takes and collects old mattresses and box springs to recycle into materials for dog bedding and dog houses and they then send the coil springs off to a metal recycling scrap yard.
The trusted inmates come out to help unload them in a designated area.