Quickly toggle plugin activation status from the toolbar.
For those times when you're too lazy to visit the Plugins screen to toggle a plugin on or off.
It's also helpful when troubleshooting:
This simple plugin is awesome, especially when it comes to diagnosing problems. One of the tenants of troubleshooting WordPress is to disable every plugin and re-enable them one by one in a process of elimination.
Generally, this is accomplished by having the plugin management page in one browser tab with the front end of the website in another. When a plugin is deactivated, switch browser tabs and refresh to see if the problem disappears. Depending on the speed of your site and the number of plugins installed, this can be a cumbersome experience.