What does "category" do?

[Lois McMaster Bujold's Blog (RSS Feed)]

So...What does marking one or more categories for one's post do? Tellingly, there is no category for books or reading, although there is one for writing and poetry, which seems a little... off-...

More... (http://blog.myspace.com/index.cfm?fuseaction=blog.view&friendID=164952151&blogID=318095820)