Why? No really ... has the vertically integrated, spiritually holistic Mac experience with Safari and/or Firefox suddenly been disappointing?
Keep the chrome on the Chevy bumpers and Denny's diner decorum please. Do we really NEED another web browser?
Yes. Chrome is the only browser so far with sandboxing technology built right in. It increases the stability and security for everybody by default by making sure that each tab is a different process and the plugins are also a different process. (IE8 might have this technology but it is unclear).
Mozilla is already getting ready a team to start the sandboxing technology for the next major version of Firefox, not 3.5. Possible the 4.0 release.
Chrome also brings for the Mac users, the sandboxing along with extensions ability to a superfast webkit browser that Safari won't have at least until Safari 5.
Competition is better for everybody, it means each browser now has to compete for users and need to add innovative features to separate it from others.