Sometimes when trying to figure out the cause of an issue on a site it is useful to see what the action(s) at or before the problem started were. Simple History is a great way to track exactly the root cause of an issue. Simple History works behind the scenes and records the exact time of many common events such as user logins (and there IP), the creation or updating of posts, setting changes and even tracks changes to some common plugins.
I know when I am having a problem on one of my sites, it often helps me to see who logged in when the problem occurred and what exactly they did without the he said/she said. Installing this plugin does not take a lot of time, is very useful and since it is small the plugin does not have any significant effect on the performance of a site.