I got the idea for Spellbook, because I was learning programming, but also having some projects that involved quite a bit of research on subjects like audio phy

I got the idea for Spellbook, because I was learning programming, but also having some projects that involved quite a bit of research on subjects like audio physics, statistics, data visualisation etc., and I wished I could keep my bookmarks in a generic hierarchy, but also attach some categories (bookmark folders) to categories for my different projects at the same time. So, I also see the value in having subcategories in addition to freely formed networks or associations.