It's an easy way to share photographs, since you can avoid having to deal with extra services like Flickr.
As with other content managers, you can modify Gallery's source code, since it's an open-source application. You're free to personalize its appearance and modify its behavior to your liking.
If Gallery stands out for any reason, it's probably for its simple control panel. The interface, no matter if you're working as a user or an administrator. Everything you do will be done in the same window.
To get Gallery up and running you'll need a web server, like Apache or Microsoft ISS, PHP 4.1.0 or later, and a database, like MySQL or Oracle.