The info plugin helps with various basic settings for the website.
Some plugins are too low level (below the editor) and thus they cannot have their own settings defined within themselves. In this case, they either have their own user interface implementation (like the users_ui) or their settings are handled by the info plugin.
So the info plugin includes the basic installation and removal of plugins on your website. It also handles the unsubscribing of mailing lists (sendmail is really low level too.)
The main information settings include things like the name of the website and how to handle the breadcrumbs, which are managed by the output plugin.