When listing any content item, it would be incredibly handy for filtering purposes if we could get the category name (or its ID, not the display name) included in the output. That is to say, for example when we get a list of entries for the categories Graphics/ Graphic Apps and Video/ Video Players, the entries in each of those categories would include a corresponding <category>Graphics/ Graphic Apps</category> or <category>Video/ Video Players</category> tag in the <content> </content> item. Alternatively, we could include the ID for the category, though the name would make things a little easier on the client side (and the output would be more pleasant to read in general with the name).
The purpose, specifically, is to allow for subcategories to be shown in Discover, but it seems like something which would be quite handy in other places as well (and the API of KNewStuff seems to expect that it happens, even if it was never put into the ocs spec). That is to say, while it is not a part of the standard, it seems very much like it's something that's been left out sort of accidentally. We could add it as meta-tag information, but at the same time this is specifically for any and all types of content so that would kind of go against the point of those meta-tags.