The PluginManager is not affiliated with wp-plugins.org
The Plugin Manager (download) is a plugin which gives you central access to all the plugins stored in its database. It also allows certain plugins to be downloaded and installed in a single click. It makes keeping up with plugin development easy by informing you if upgrades are available.
For plugin developers, adding your plugin to the plugin database has two main advantages. It gives people easy access to your plugin (particularly if you package it for 1-click, a pretty painless procedure in most cases), and also lets you easily keep users informed of updates, as they can check for updates for all the plugins in the database in just one click.