I personally needed it to delete dated/forgotten plugins that posed a security risk by keeping around. Having a large number of different sites in a network, it's sometimes hard to determine which plugins are being used where. This plugin clearly highlights which plugins are not being used at all.
"...if you are not using a specific plugin, delete it from the system." http://codex.wordpress.org/Hardening_WordPress#PluginsWhen wordpress network installations get large and you want to know what's where and what's not needed, this plugin is the simplest solution.
No, just go to "Plugins" in the admin.