Most of the internet users maintain multiple Google accounts which serves various purposes to them person mail, business or something else. Google conventionally does not allow using more than one Google account at the same time in the browser. So you need to do log-in and log-off again and again.
Now there is an official way to link different Google accounts; So that you can use various Google services like Gmail, Google Reader, Google Voice and others with just a click of the mouse within the same browser.