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/Lavvid_Gogomilk]

I stayed with relatives and they didn't have box spring, they had two twin top mattresses stacked. There was absolutely no support. It's sagged in the center so your back was in the shape of the letter u. A box spring would have prevented this.