Web browsing on the PC, whether a laptop or desktop, is accomplished using a pointing device – such as a mouse. Most mobile phones feature a four-way rocker switch or joy-stick which allows you up, down, left, right and "select" or "OK." This kind of navigation is especially problematic in arbitrary two-dimensional layouts such as the one below:
How to allow a four-way navigation to navigate an arbitrary two-dimensional layout has been up to each browser implementer – the result is that there is now a variety of navigation methods out there. While variety is the spice of life, in this case it blocks adoption of a technology because there is a new learning curve for users for each device they pick up, to access the same content or service. The problem becomes even more acute when in the context of navigating a two-dimensional space the user can navigate in to an interactive SVG space (sometimes rendered by a browser plug-in and sometimes rendered by a browser itself) which itself may have focusable items and then back out into an XHTML-defined space.
The red arrows in the above diagram illustrate a standard way to implement this. The CDF specification specifies this in detail and thereby sets a bar for browser vendors – thus enabling a more predictable user experience across different devices and browser implementations.