The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!).
The real purpose for the constellations is to help us tell which stars are which, nothing more. On a really dark night, you can see about 1000 to 1500 stars.
Trying to tell which is which is hard. The constellations help by breaking up the sky into more managable bits. They are used as mnemonics, or memory aids.

A constellation is a group of stars that are considered to form imaginary outlines or meaningful patterns on the celestial sphere, typically representing animals, mythological people or gods, mythological creatures, or manufactured devices. The 88 modern constellations are defined regions of the sky together covering the entire celestial sphere.

Origins for the earliest constellations likely goes back to prehistory, whose now unknown creators collectively used them to related important stories of either their beliefs, experiences, creation or mythology. As such, different cultures and countries often adopted their own set of constellations outlines, some that persisted into the early 20th Century.
Adoption of numerous constellations have significantly changed throughout the centuries.
Many have varied in size or shape, while some became popular then dropped into obscurity.
Others were traditionally used only by various cultures or single nations.

The Western-traditional constellations are the forty-eight Greek classical patterns, as stated in both Aratus' work Phenomena or Ptolemy's Almagest � though their existence probably predates these constellation names by several centuries. Newer constellations in the far southern sky were added much later during the 15th to mid-18th century, when European explorers began travelling to the southern hemisphere. Twelve important constellations are assigned to the zodiac, where the Sun, Moon, and planets all follow the ecliptic. The origins of the zodiac probably date back into prehistory, whose astrological divisions became prominent c. 400 BC within Babylonian or Chaldean astronomy.

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