It really isn't. Although market share is always nice, the real reason Google made Chrome becomes obvious when you look at the javascript benchmarks compared to all of the other browsers. It blows them out of the water. Google wants to be able to do as much as possible from the browser so that it can create increasingly complex web applications that attract viewers to whom they can advertise. To do this, it needs lightning-fast javascript abilities. This is the reason Chrome is open source: to give this javascript interpreter away for free.