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. These features don't do much for me, but they might be useful to somebody. Ok, tell me the negatives.
Hamster has to fetch the article bodies.
This means you will need a lot of disk space if you want to work with multiple groups of any size, or with binary groups. Regarding mail, it won't do HTML or MIME, at least I don't think so. Some of this may be Gravity's limitations but some is Hamster. So, if you rely on mail with a lot of rich text and HTML you will be disappointed. These instructions are only to get you started.