Why do telecommunications satellites need to be in geostationary orbit?
So that they stay (apparently) roughly over the same spot of the earth as it rotates. If they didn’t, then they would keep disappearing over the horizon and therefore be unavailable for most of the time. If they are geostationary they are available all the time. Although they are theoretically stationary, they do in fact jiggle around a bit as they and the earth rotate, depending on the parameters of their orbits. Generally they describe a small “figure of eight” in the sky, so comms dishes using them often have to jiggle a bit to keep them in view. The reason your Sky dish is fixed and doesn’t have to jiggle is because it has a sufficiently broad field of view not to have to, but the cost of that is lower gain.
To enable a continuous communication between ground stations. In the 60’s Telstar was a communications satellite that was used to communicate between USA and Britain. But unfortunately it was NOT in geostationary orbit, so the link dropped out after a few hours until the satellite come back to a position where the link could be re-established. This problem was highlighted when the TV progran “Tomorrows World” lost the link during a live broadcast.
