A plug-in that lets us change Apache settings without having to do too much (i.e. more or less a way to set one line or more in Apache from the server.)
For example, a customer may want to use www. in front of his domain and enforce it. So if someone tries to access http://example.com instead, we can ask Apache to add the www. which makes it a lot faster than asking the Snap! server to do it.
Another example of such a redirection is for systems such as feedburner. You can setup your Apache system to redirect everyone from your default rss.xml file to read the feedburner feed instead. That way you offload all the transfers to feedburner (i.e. Google) instead of your website! This includes a test to make sure that when feedburner itself accesses that file, it reads it from your site directly instead of redirecting to itself.
Actually, all static (that won't ever change nor be removed) redirections from a path to another should directly be done in Apache (301 redirects.)
Some plugins or even core may add rewrites of the input path to better fit the backend needs. For example, accessing a file could be done with a /files/ folder, but internally require /uploads/files/.
This means the core system of a website creates its own Apache configuration file. This is particularly practical if we want to avoid having users create an Apache entry (however, it's very specific to that one web server.)