Chromebook's are a project by google to produce laptops that are specifically used for the internet. Without the internet, the computer is very much rendered useless. The computers are made very cost effective, because all of your applications belong on the internet. The computer is more or less optimized for only the use of a web browser.
Because I can.
Also to prove to others that something like this is easy enough that I can do it, and if you met me before you would understand just how easy this is.
Also to show jake that he doesn't need to put linux on his super awesome laptop that he has. He can just grab an arm computer.
Having a full fledged OS on something this cost effective allows anyone to have a decently made laptop for cheap. With enough knowledge of GNU/Linux, this laptop could be the only one you would ever need.
Honestly, you never need to boot into chromeOS in the first place.