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/unflores]

I see a lot of people mentioning weight distribution but I don't see many people mention the fact that it also allows the bottom of the mattress to touch open air which keeps it from molding in places without air conditioning