As time passed, the sky became filled with constellations, many of which included the same stars. The International Astronomical Union stepped in to gain control and make sense of things in 1925. They adopted 88 official constellations and assigned areas of the sky to specific constellation names. It should be noted, however, that they made no requirement that the constellation be easily seen nor that the legends associated with the constellation make any sense. In fact, most constellations don't really resemble the creatures or characters they are named after.
Constellations can be a useful way to help identify positions of stars in the sky.
The patterns we see are for the most part just by chance. The individual stars in a constellation may appear to be very close to each other, but in fact they can be separated by huge distances in space and have no real connection to each other at all. For example, look at the image below of the stars which make up the constellation Orion. At different times of year, different constellations can be seen in the sky. Different constellations can also be seen depending on where you are on Earth.