The way I see it, a category allows for the free-flowing discussion in a central place about a specific area of knowledge like @Sze had in mind. Tags tend to be more of a cross-cutting concerns nature, where you can have posts in very different categories all sharing the same tag. The Help category in a programming language forum is essential and will gather up most of the posts, but should be mainly for actually helping someone that’s currently stuck because of something they don’t know; more of an “I’m stuck because I don’t know this” versus “I would like to know that”. In reality, the lines are not precisely defined, every time you’re dealing with taxonomies and information architecture, you’re bound to find gray areas with overlapping boundaries.
So we could have a Zig Internals category and a zig-internals or compiler-internals tag at the same time. With the category you get a one-click listing of specific discussion in that area, and with the tag you get filtering in search across all categories. Could be the best of both worlds. 
As in many areas of technology, the weakest link is the user; if they don’t select the appropriate category or assign the right tag, the whole scheme comes crumbling down. But our community has done a pretty good job up til now and the moderators and I try to re-categorize and tag whenever we can.