Why shouldn't you just call a few local agents to find the best rate? Because it might costs you more than you think. There are two big problems with buying insurance this way. The first is that you are dealing with a commission driven insurance agent. It's not the typical commission structure that you might be use to when dealing with a normal salesman, it's actually worse. Many agents are paid by how much they can up sell you on policies you may not need. Forget the normal salesman trying to sell the loaded BMW when all you want is a Chevy, these guys want you to buy car with a houseboat attached to the back. The bigger your policy, the more they make and like most sale people, they probably have dollar signs in their eyes as soon as you walk through the door.
The second issue is that you might be missing out on an insurance company that isn't locally listed. Even if all of your phone number searches are done on the internet, a search for "car insurance Town A, Your State, USA" might not turn up results for the agent that is located in Town B, Your State, USA. So even if you find the "best deal in town" it might not be the best deal at all. Also keep in mind that local businesses often form an agreement with each other not to sell below a certain dollar value. So, they could potentially be gouging you without you even knowing it.
So how do you find out who has the best auto insurance rates? Well, the internet is loaded with companies that will compile a list of insurance companies that serve your state. Many times these list are organized by rate and incredibly easy to use. One example for me was after Tropical Storm "Faye" this past summer. My car was totaled by water damage and after my former insurance company paid for my car, they dropped me. While searching locally, I found that most of the rates for my car were about 10-15% higher then through the compiling website.