Before the advent of plugins, the functions that are now plugins were part of the template-functions.php file or in somecases, in separate php files. So, they were loaded each time someone viewed your gallery whether or not you used the function. That cost resource unnecessarily. With the implementation of plugins functions are divided between "standard" and "optional". The optional ones being those that are not always used by themes.
There are two steps to using a function which is now included in a plugin. First, the checkbox for the plugin must be checked enable. (A few are checked by default--ones that were part of template-functions.) Besides enabling the plugin, of course, you must actually make use of the functions it provides.
I think we will eventually include a link to the function documentation of the plugin within its description. First, though, we have to generate that set of documents.