Every Android user should know that Android is based on Linux by now. Android start off from Linux, got optimized for mobile use, and evolved in its own way. But now Ubuntu helps to bring the full desktop experience back to Android, so users can enjoy Android, them mobile OS while it's only on their phone, but the Ubuntu OS when connecting to a desktop monitor.
This idea really isn't new, and perhaps the most public appearance of such an idea was with the Motorola Atrix and its docking abilities, that would show a Linux-based OS when docked to a laptop shell. But Motorola's implementation was far from perfect, and Ubuntu's idea of putting this capability in many Android phones makes a lot more sense.
Ubuntu is also the most popular and consumer friendly Linux distribution out there, so this shouldn't be a drawback at all for most consumers, who just need to do some regular Office or development work on the PC, because Ubuntu has them covered. Plus, Ubuntu gets to enhance the phone experience, by making everything that is on the phone available while on the desktop mode, including contacts, but it also syncs pretty much everything you do in the desktop mode with the phone.
Google is rumored to do something similar with Android 5.0 and Chrome OS, but personally I think using a dual boot strategy with Chrome OS would be a mistake, because people might prefer to even use Ubuntu in this way than Chrome OS. I think for Google it makes more sense to just allow Android to show the tablet UI, perhaps even more optimized for desktop use in this case, when the phone is docked. Then you at least get access to all Android applications, and not just the web.
Plus, if you'll have Chrome for Android, then what is really the point of having Chrome OS alongside Android, and be limited to only Chrome when in "desktop mode" ? We don't know if they will really go this way, but if they do, hopefully they have a strategy that makes sense, and they aren't doing it just because they have to find some place where to put ChromeOS.