I can't think of any topic in abstract algebra that doesn't have a tag in itself.
When should the abstract algebra tag be used ?
It's probably more common for users to follow a handful of general tags rather than lots of specific ones, at least when they're new. (This was certainly true for me). So this is one possible reason for having a general tag like abstract algebra on a question, as well as maybe some more specific ones.
I do follow "subtags" of abstract algebra if I notice an interesting question that didn't have the more general tag as well, but it's probable that a lot of these questions entirely slip past me.
When one is too lazy to look up the more specific tags. /rimshot
All kidding aside, I see that tag as largely a vestige of the old days when it was only important to differentiate between algebra and algebra. But every now and then you do get questions like this and this which don't really want any of the more specific tags.