We need to specify the position of an object in the sky to point our telescope at it. Only two coordinates are needed to point a telescope. For example, only the altitude and azimuth are needed to point an alt-azimuth mount telescope. There are, however, a number of different astronomical coordinate systems in use.
Astronomers use the most convenient one depending on what they are trying to accomplish.
The Horizon Coordinate System is a simple way for describing the position of a celestial object. There are two main disadvantages to describing an object position using Horizon Coordinates. One is that as the Earth rotates, objects move in both altitude and azimuth . Another is that the coordinates you determine are only good for your location. For one observatory to use coordinates to an object that were generated at another observatory, a calculation would need to be made to adjust for the position on the Earth of each observatory.