In contrast a hierarchical tab structure can act both as the stuff I'm currently working on as well as a way to store all things I find interesting in named groups. If you didn't manually tag all your bookmarks, ideally with a consistent naming scheme, finding certain sites again can be quite a task. But I know what tab group a certain website that I last used 2 years ago and can't remember the name/url of has to be in. It's basically the web version of a file manager. Why would I spend time to organise bookmarks in some folder structure if the structure already exists in the tab bar, always up to date?