By doing so, you can still get your mails, notifications, and so on but without draining your battery by having wifi constantly turned on.
Also useful if your data plan is expensive. Simply turn it off and let AutoConnect use wifi while saving your battery. Same if you don't have a data plan.
AutoConnect also features a scheduler so that you can restrict the auto connection times (only in the full version).
Check the website for the complete documentation and support.
Similar applications: Y5, Juice Defender, Tasker, Auto Wifi Toggle, Wifi Switcher, Green Power, BatteryFu, WeFi
此版本中的新功能：added Russian translation (courtesy of my wife)