Here is why. Once you begin using Dropbox, you become more and more indifferent to the hardware you are using, as well as the operating system on that device. Dropbox commoditizes your devices and their OS, by being your “state” system in the sky. Storing credentials and configurations of devices, and even applications are natural next steps for this company. And the further they take it, the less dependent any user becomes of the physical machine (HW and SW) that is accessing that data (and state). Imagine the number of companies, as well as the previous paradigms, this threatens.
That is a major, major deal. And it comes at a time where there are many competing platforms on both desktop and mobile. This “unsure” market backdrop ensures the need for a cross-platform solution and plays right into Dropbox’s hand. You can lose your desktop computer, you can lose your smartphone. It doesn’t matter, because all you really care about is in the Dropbox cloud.