The purpose of a constellation is to enable to viewer to see and remember the entire position of the night sky; both the rotation and the current position.
The constellation symbols are mnemonics, taking advantage of our natural ease for remembering symbols and stories. It is easy for us to remember a small pattern of stars related to a symbolic figure, such as an animal or a hunter because these symbols are already familiar to us. Knowing only two constellations the viewer can understand exactly which part of the night sky they are looking at with only a glance. Without the constellations one would have to attempt to identify a haphazard array of millions of dots, which would be next to impossible, hence the necessity for constellations.
Constellations tell us where we are in space, and from that they also tell us where we are in time. The ‘ancients’ [sic] believed that history repeats itself, and worldwide events, such as catastrophes and the rise and fall of empires, could therefore be predicted by tracking the constellations over thousands of years. Modern astrology is a corrupt and perverted form of these, once highly respected, ancient practices.