Categories can be tags, sure, but not all categories are tags, and not all tags should be categories. I think of categories as a table of contents and tags as the index page of a book. If I’m searching for a broad topic, unsure of exactly what I need to find or the keywords, then I will hit the table of contents (categories). If I know the exact word I need in order to find the information I want, then I will hit the index page (tags).
This seems reasonable, and I will start to use tags and categories differently from now on.