An ordinary telephone signal has a bandwidth of about 5 kHz. These signals are then superimposed on a "carrier" signal that can be in the mega or giga-Hertz region. Now if your carrier is restricted to frequencies, say, between 4 to 5 megaHz then your total allotted bandwidth is 1 megaHz. That would be equivalent to 2000 telephone conversations. Actually less, because there must allowance for spacing between adjacent channels in order to eliminate interference.
Television signals are bandwidth hogs. A typical video signal can take up to 5 to 10 megaHz bandwidth. So in a one gigaHz band slot you can only cram in 10 to 20 of these. If you try to stuff more in you will wind up with a blivet (American slang).
So, freeing up some of the old analog TV signals and reassigning them to telephone conversations allows a vast increase in the number of cell-phone channels available.
p.s. You may also be asking: why not just go up and use higher and higher frequencies to extend the total bandwidth? The behavior of very high frequency electromagnetic waves starts to become quite different. Visible light, ultraviolet light, x-rays, and gamma rays, don't interact with matter at all like radio waves. Visible light can be used, but requires optical cables to transmit. Higher frequencies are essentially useless for broadcasting signals. Among other problems, they usually are absorbed when they hit normal matter.
(published on 03/05/12)