Hamster will fetch the articles. Gravity connects to Hamster just like any other server. Hamster can pull from multiple servers and keeps the articles in one list, which is an advantage for readers that cannot handle multiple servers. Thanks to John Moreno, who suggested that Gravity might be able to read email with Hamster. Most Gravity users will have no need for it.
Gravity handles both multiple news servers and off-line reading quite well.
Like I said, the main use I can see is for threading email. I have a group for each mail list. I also use Hamster for testing rules and regular expressions. Another reason why you might want it is that you can keep groups from multiple servers in one list. You can also post to different servers.