Spectrum matters because communications matters, and wireless and mobile communications now dominate all communications globally.
Spectrum sharing matters because communications spectrum is a scarce asset, and demand is growing very fast, both because billions of new Internet access users will come online, and because new Internet apps and devices consume vastly more bandwidth.
There is, for example, almost no uncommitted communications spectrum available in the sub-2-GHz range.
Today, the U.S. government, for example, possesses almost 60 percent of radio spectrum and possesses over half—1500 MHz—of the valuable 300 MHz to 3 GHz spectrum useful for terrestrial wireless and mobile communications.
Much of that spectrum is lightly used or even not used. At a time when most observers believe people, organizations and businesses will need vastly more Internet and communications capacity, that is a waste of scarce resources.
Though there is an expectation that much spectrum in millimeter bands (3 GHz to 300 GHz) can be allocated for communications purposes, most of that spectrum will be severely “short range,” and hence best suited for indoor or small cell applications.
The promises of spectrum sharing is more efficient use of communications capacity with the best propagation characteristics, suitable for longer-range communictions.
Somewhat ironically, "sharing" long has been a major pattern for communications usage between a couple hundred MegaHertz and 300 GHz.