I don’t know where the separation in blog organization began that left us with categories and tags. I know that in human language, there is a difference between the two.
But on web sites, the difference between categories and tags is defined by the way the writer or writers use them.
For me, I’ve never seen much of a difference between the two: Both are ways of automatically creating sections of a web site that consist of posts with like content.
There have been times when I’ve used categories but not tags. Maybe I’ve done it the other way around. I can’t remember. And it’s not that important.
These days I use both categories and tags when both are available, and while they mirror each other for the most part, I generally go deeper with tags and add a few more to a given post.