[AntonSafonov]

Folders die immediately when you need one more way to classify your files. E.g. you have photos grouped by date, but want to add genre or group by people on the photo. The same with books - you can have them in one big list, because very often you can't really select the primary system for categorization, since one book may cover many different topics, so you may want to have tags for every major topic, and then also by year and by author. Plus some tag to mark books you've already read or planning to read.