Not sure why you think I don't know why they exist, but Chromium is quite different from Chrome: https://code.google.com/p/chromium/wiki/ChromiumBrowserVsGoo...
Except for the color of the icon and the lack of support by default for AAC, H.264 and MP3 (I have support for those 3 in Chromium in Arch), do you find anything relevant that will make some non-expert notice they are using Chromium and not Chrome?
I hadn't even noticed Chromium doesn't have support by default for those codecs as I have used HTML5 audio and video without an issue. Really, for a non-expert Chromium is just Chrome with a blue icon.