Question

Why is being geosynchronous important for telecommunications satellites

Answer 1

Because any antenna on the ground that uses a satellite has to stay pointed at the satellite.  

If the satellite is moving in the sky, then the antenna has to keep moving to follow it.

But if the satellite APPEARS to be motionless in the sky, then the antenna on the ground can be aimed once, and it doesn't have to be moved after that.  "Set it and forget it."

There would be no TV satellites if everybody who wanted to use it had to keep moving their little dish to keep the TV picture coming in.