There are several reasons why the advent of millimeter wave spectrum for 5G vastly increases bandwidth, and thereby creates new business opportunities for mobile operators. Not only will millimeter wave spectrum represent a vast increase in mobile capacity (an order of magnitude to two orders of magnitude effective new spectrum), but millimeter wave spectrum also is more efficient.
Where spectrum below about 2 GHz has a spectral efficiency up to 2.5 bits per Hertz in a 4G context, and up to 3.8 bits per Hertz on a 5G network, millimeter wave spectrum has an efficiency up to seven bits per Hertz.
Basically, not only does millimeter wave spectrum represent an order of magnitude more capacity (Hertz), it also represents more bits per Hertz, as much as double what is possible on a 5G network using spectrum below 2 GHz or so. The reason has much to do with frequency and its relationship to symbol representation.