system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some flat trailers carried by big rigs arched?

Got stuck behind one in traffic today and couldn't figure it out.
gpt
It increases the load capacity of the trailer.   If a similar trailer were to be loaded with the same payload, but it was flat, it would flex downward slightly.  Think of it like a leaf spring, they are arched upward to support weight.

You might also ask why they don't put coiled up springs underneath the middle if it helps carry heavy things, but the answer boils down to design simplicity.