It is a way for us to try this Google OS output without having install onto PC or what.. so you have no worry about losing your important files or something. Or, you can also use a kind of virtual machine Virtual Box or VMware, that's it.
If someone has a PC in the house, and notebooks to the office or college, as well as communicating via mobile phones. And the movement that later happened is that people start thinking how to unify everything in one hand. Actually it has started to be fulfilled with the emergence of a PDA/smartphone, where one can summarize all the computing needs in one hand. And development is what makes Google's ambition to dominate this segment.