It's basically going to allow you to dial quickly without having to open up Safari, go to the GV page, pick out a number and hit call. An easier way, if you're calling mobile numbers, is to use the 406 number associated with that phone. To get that number, text the person with your GV, then wait for a response. The response will come from a 406 number (if that person isn't also on GV), which you can save into your address book to use for that person from now on. Calling that 406 number will route your call through Google Voice.
But it is pretty useful for landlines, since landlines can't text back. [Ironic Sans]