I don't think your summary does Google Voice justice. Within their database they have phone numbers with the area code 406 dynamically assigned to a Google voice subscriber and each of that subscriber's contacts. This means that if you call from your phone which is registered under Google voice the 406-xxx-xxxx that corresponds with a certain person within your contact list, you will be in turn calling that person transparently, and they will see on their caller id your GV number not the actual number of the phone your dialing from. This gets around the whole "Hey everybody! I have a new phone number, don't use the old one" problem as well as the manual proxy that is calling your own GV number from your phone then putting in the number of the person you want to call. Since the 406 number is personal to you and is dependent on you and who you are calling, Google only needs to buy enough 406 numbers from a Telco provider to fulfill the person has the largest contact list.
It is hard to describe but, in effect, they have reduced the need to know phone numbers because they do it all in the background transparently, as well as the manual proxy annoyance.
The downside is that it is not fully implemented. To see one of your contact's own 406 number, you have to SMS them from your GV number and they need to reply back. There is a rumor that Google may be automating this soon.
If you don't think about the security concerns, Google's got a winner here