Speaks for itself. I don't think it's arguable that to be good at this game, you need to keep in mind a truckload of information. From how to build your character to what mandatory weapon types to pick up at what levels, there's plenty to keep track of. Heck, sometimes you just discover some cool tactic in a dungeon, but you know you're going to forget by the time you go there again. I myself always have trouble remembering everything when I return to the game from a month or longer hiatus.
Now I'm not the kind of person who alt tabs out to check the Wiki for everything. (Seriously, who does that? That's asking for memory leaks.) Plus the Wiki isn't exactly complete either.
I'd love to have a notebook option on the character sheet, where you have several pages to leave notes for yourself. If storage is a problem, make it clientside only.
Sure we could have an actual notebook in front of us IRL, but let's face it, in this day and age, it's just inconvenient, compared to text you can edit on a whim.