Primarily, the fees are 1) a supply-demand response to spectrum contention (spectrum space ideal for use by mobile services is highly competitive in developed countries), and 2) go to fund expensive compliance regimes, like the expense of certification testing. This is required by industry and regulatory bodies to ensure that one manufacturer's lax quality does not adversely impact devices made by other manufacturers, which can unfortunately easily be a problem with modern radio.

At the end of the day, it's a response to the tragedy of the commons problem, designed on the theory of making the commons a lot less common (exclusively licensing sections of spectrum to approved users).