system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is a box spring for?
gpt
For bed frames that do not have supports across the middle.

Some bed frames are just the outer rectangle connected at the head, foot and maybe once across the middle. Set a mattress on that by itself and when you sit on it it'll push through the frame. Putting a sturdy box spring on it first prevents that.

If you have a platform bed frame or bed frame with sufficient support in the middle then a box spring is not necessary.