It's not clear to me that the government mandated the change because it wants the spectrum. AFAIK, the amount of spectrum allocated to commercial television will remain the same. Digital is a more efficient mode than traditional analog, so the result is the possibility of more broadcast channels in the same amount of spectrum. Government does benefit indirectly via additional regulatory fees, taxes, etc. Broadcasters benefit more directly by having effectively created more real estate to use up. The FCC or whatever they're called these days has always assumed the ability to regulate the airwaves, and generally has done so with the guidance and lubrication provided by broadcasting interests. What's happening now isn't really anything new, and has happened before with television, FM radio, etc.
Of course, this DTV stuff is an upgrade that no consumers really asked for. The changeover has been scheduled for several years, and has been handled in the usual graceful way that we'd expect any government mandate, with things like converter boxes effectively just hitting the shelves now, confusion, questionable benefits to the end user, and government handouts not only to special interests but to consumers as well.