Well, this allows a bunch of people, not necessarily involved with the Maven project, to come along and build some plugins. They are free to look after just that plugin, though I hope that they'll get involved in the community of people here and work on other plugins as well as help maintain the common infrastructure like the site.
Maven plugins may also be developed here because they are not compatible with the Apache license. Plugin authors may choose to use other licenses (BSD, GPL, MPL ...) to license their work or may be forced to because of the nature of the plugin and its dependencies. These plugin authors are responsible for working with their plugins license but the mailing lists are a great place to get help if people get confused on what can or can not be done within the mojo project.
The sandbox is used to "incubate" new plugins. Sandbox plugins will not be released to the repositories and if you are interested in trying it out you will probably have to build it from source.
All new code should be put in the sandbox so we can make sure that the plugin uses the correct code style, that is has proper documentation and generally has the expected quality.