In paper based days we spoke of ‘filing’. A good bit of the art of good filing was determining what folders to create, and which folder to put each document into; this impacted how easy it was to find something. When computers came along we gained hierarchical folder structures.
When the internet came along, we could get pages by other methods, besides their location: first there were hyperlinks and then full text search of all documents. But in a cut and paste world, text based searches can return an overwhelming number of documents.
Tagging is now common, and it is very powerful. Many pages are associated with more than one thing. Before computers, a good secretary was one who could make good choices about which folder to put each document. But with tagging this is not a problem, a page or document can have more than one tag. Beware, you have to determine how fine grained your tagging system is. Too many tag names with similar meaning can result in miscategorization, defeating the rational of all those tag names.
Bookmarks and blogging about what you see or do is another way of keeping track of pages. WP allows tagging of posts but not pages? Why! How do we work around this?