One of the main reasons to integrate the news transport layer into the newsreader is to allow greater control over what articles to download. There's not much point in downloading huge amounts of articles, just to find out that you're not interested in reading any of them. It's better to be somewhat more conservative in choosing what to download, and then mark the articles for downloading manually if it should turn out that you're interested in the articles anyway.
One of the more effective methods for controlling what is to be
downloaded is to create a category and then assign some (or all)
groups to this category. Groups that do not belong in any other
category belong to the
default category. Gnus has its own
buffer for creating and managing categories.
If you prefer, you can also use group parameters (see Group Parameters) and topic parameters (see Topic Parameters) for an alternative approach to controlling the agent. The only real difference is that categories are specific to the agent (so there is less to learn) while group and topic parameters include the kitchen sink.
Since you can set agent parameters in several different places we have a rule to decide which source to believe. This rule specifies that the parameter sources are checked in the following order: group parameters, topic parameters, agent category, and finally customizable variables. So you can mix all of these sources to produce a wide range of behavior, just don't blame me if you don't remember where you put your settings.