How did people find out about constellations?
They didn't find out about constellations...Long time ago, it was easier to remember certain stars by grouping them with others, and naming that constellation to what the picture looked like (stars connected). I may be wrong, but this is what I've been taught. It was a long time ago, so if anyone has a different answer, or agrees with me, please post away :) !
It is an easy way of identifying groups of stars and labelling and naming stars within constellations. Many constellations are long established. So they are also commonly known by people, making it easier for people to understand what is being talked about, be they scientists or ordinary people who can identify constellations and stars.