It keeps your database much cleaner, and faster, by limiting the amount of past versions of a post that are allowed. This can cut-down hugely on the amount of data in the database of a Multisite. It also lets you change the frequency of auto-saves. The WP default is a pretty short amount of time. Increase it a little for less load on your server, and so auto-saves don't quickly fill up the amount of revisions you limited it to, with the 1st setting.