Google, nor anyone else, can change your caller's phone service/plan. So no, it does not allow people to call it for free. People who call it do so using their own phone and service plan.
You can, however, get a GV number in a different area than your own. For example, if you have friends/family in Houston TX, you can get a Houston TX number so calling it would be a local call to the people in Houston.
The purpose of a GV number isn't really any different than the purpose of any other number. It is to make and receive calls. Google gives you some unique features such as enabling you to route incoming calls to different phones. The OBi lets you use your house phone to directly dial and receive calls via your GV number.