That's a perfectly reasonable question.
Bench planes are used for taking rough lumber and squaring and smoothing it for use. If you only buy wood from the hardware store you're used to seeing "S4S" or "surfaced four sides" boards, so it's already been squared and smoothed by machines. If you buy your wood from a specialty wood dealer, it usually comes rough. In both cases it's usually not really square and straight, so you'll need some means to remove warp, twist, cupping, etc.
Hand planes are the old-school tool for the job. Longer planes flatten longer boards, and shorter planes are used to smooth and clean up after the rough work from the earlier planes. I'm in danger of just recapitulating this article by Chris Schwarz, so I might as well link the whole thing:
https://www.popularwoodworking.com/wp-content/uploads/CoarseMediumFine.pdf
This is an excellent explanation of which planes you need, what they're used for, and how to set them up for that use.