The problem in english is that “for” can have multiple meanings in this case. It can mean the product is just meant to be used on item X or it can mean it intended to make the product more like X.
In this case, I’m confused because some people with curly hair hate it and want it straighter, so “for curly” hair might make it straightens or at least not make it curlier, while people with straight hair who DO want it curly would want a intentional product “for curly hair” to make their hair curly.
Which is it?
Generally speaking, products labeled for curly hair are gentle (because curly hair is often delicate) and designed to help reduce frizz and help with definition.
So it won’t make straight hair curly, no. It will help curly hair look “curlier” because less frizz and better separation of the curls makes it look curlier, but it doesn’t literally tighten them, may even relax them very slightly.
So they are for people with curly hair, to improve the look and/or health of curly hair.