The dip tube is part of the cold water input.
It directs the cold to the bottom of the tank so that you still get the hot water out the top while refilling.
[url=http://homerepair.about.com/od/plumbingrepair/ss/trblsht_hwh.htm]Troubleshooting a Gas or Electric Water Heater - Hot Water Heater Repair[/url]
Take a look at "cutaway photo" near the bottom of the page.
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