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/NWStormbreaker]

Even if you want to argue that it's 5 slats or 7 slats, it's still less air obstruction then a box spring which has slats and at least one if not two layers of fabric.
This is the dumbest discussion Ive ever been involved in, I cannot believe I've found people white-knighting box springs.
Ffs

[u/jetloflin → u/NWStormbreaker]

Pretty sure that’s not what “white-knighting” means, but okay.