Vivaldi is not just another repackaged Chromium, it has it's own completely custom UI and special features. For example, it has built-in mail, rss, calendar, and contacts "apps", and a built in screenshot tool that lets you capture an entire page or a single element.

Features like that are why you would want to use Vivaldi over another Chromium-based browser. I don't even know if they actually went through the effort of a thorough de-Googling. Also, their custom UI is not open source, so the browser is partially proprietary.