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?
Products for curly hair are designed to minimize the negative effects of curls (such as frizzyness) while attempting to accentuate the curl.
I don’t think I’ve ever seen a product for straight hair other than maybe a straightening iron (and mists, etc. designed to be used before/after ironing), which does indeed iron your hair in an attempt to make it straighter. Edit: straightening your hair with an iron only lasts until you wash it again (or it gets wet by other means, including heavy humidity).
There are two exceptions to this: a permanent (“perm”) solution does actually make your hair more curly. Despite being called permanent, it only lasts a few weeks or months. The opposite of this is called a hair relaxer. It’ll make your hair more straight for a similar amount of time. Both of these are more harsh chemicals, and people generally go to a salon to get them done professionally. There are also home versions sold.
In summary, read the entire package before you use it on your hair.