Hi. That is a very good question. People have used constellations for many different reasons. And these reasons have changed throughout history.
Astronomy is the oldest science. This is because even the earliest cavemen would look up at the sky and wonder about what makes it run. People saw that the motions of the stars were regular and predictable.
The first use for Constellations was probably religious. People thought that the Gods lived in the heavens and that they created them. Many cultures believed that the positions of the stars were their God's way of telling stories. So it seemed natural to recognize patterns in the sky, give them names, and tell stories about them. We inherited the names for our constellations from the Greeks. And they named the constellations after their mythological heroes and legends. So behind every constellation there is a story. For example, to the ancient Greeks, Orion was a great hunter. He was the son of Neptune (god of the sea). But the same stars were considered to depict Osiris by the Egyptians. Each different culture developed their own interpretation.
A more practical use for constellations was agriculture. Before there were proper calendars people had no way of determining when to sow, or harvest except by the stars. Constellations made the patterns of the stars easy to remember. The ancient peoples knew for example that when the constellation Orion started to be fully visible winter was coming soon. Or they could look at the Summer Triangle to know when Summer or Spring were coming as well. The stars allowed farmers to plan ahead and form agriculture, and constellations made it easier to recognize and interpret the patterns in the sky.
The constellations also helped with navigation. It is fairly easy to spot Polaris (The North Star) once you've found Ursa Minor (Little Dipper constellation). One can figure out his/her latitude (North/South) just by looking at how high Polaris appears in the night sky. This allowed for ships to travel across the globe. It allowed for the discovery of America, the spread of European culture, and civilization as we know it today.
The constellations have a practical purpose today too. They determine how stars are named. When astronomers go to conferences they like to share their research with others. And usually they will want to tell someone which stars or objects they may be looking at. If they just give the coordinates (numbers) the other person is not likely to have an immediate idea of where the star is located in the sky. But if you say that the star's name is Alpha Tau then you will know that is the brightest star in the Taurus constellation. The stars are named based on the constellation they are in (all stars are in some constellation). The naming goes from brightest to dimmest star and is designated by the Greek alphabet. For example Beta Ori is the second brightest star in Orion (also called Rigel).
This page updated on June 27, 2015