Better paper that bleeds less and is less likely to get ripped through by a sharp mechanical pencil. Better binding - spiral notebooks are super cheap but if they live in your bag a while the spiral starts getting crushed and tries grabbing ahold of anything else in your bag, or the books next to it when it's filled and on the shelf. Hardbound books can make it a little easier to write in them sometimes.
Also having a really pretty notebook that looks cool can be a small source of pleasure. I like to use ones with covers styled after elaborately-bound old books, I feel like a wizard taking notes in their grimoire every time I take them out, and that's worth something IMHO.
Surely there is something in your life where you don't buy the cheapest, shittiest version of it possible, but spring for the nice version instead. Why do you make this choice? Probably some combination of the pricier one working better, being more durable, looking cooler, and acting as a status symbol.