All design matters. But design that is functional matters to me for a specific reason.
Function is an embodiment of an idea. And when the function is used the designer and user share that same idea. Not a diluted idea, reshaped by subjective language, but a perfect one. Captured in the form of the function.
If you think this sounds like mumbo jumbo consider mathematics, algorithms and functions that describe how something happens. Most people never realize just how exact the representations are until they program games for computers. All of a sudden all the dull math creates engaging experiences of movement colors and transformations.
A design can be just as exact. Consider how you use scissors, the designer of the original function had an idea about how cutting surfaces and how the movement of the hand could work together. The idea is perfectly translated in the function.
This is why design is so inspiring for me.