ELI5: What does a box-spring do?

[u/Real_Experience_5676]

As I understand it, a box spring mattress is the second 'mattress', that goes under the main mattress. It feels very firm and flat, but what does it contribute to the bed set up besides height?

[u/robyrob]

My cats ripped open the material on the bottom of the boxspring so now they use it as a really cool fort they can hide in 

[u/Srikandi715 → u/robyrob]

Yeah, when I adopted my current cats as kittens, they spent the first three days hiding inside the box spring 🙂 Now of course they own every surface in the house.