See which plugins are and aren't being used on which sites in a WordPress Network
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#Plugins
No, just go to "Plugins" in the admin.