People must have got tired of standing around or sitting on the ground and getting their bottoms wet and dirty which is why chairs might have been invented. Or may be to sit a level higher than the others and show a kind of hierarchy they were used. But whatever the use, chairs have been around for centuries now and the first example of a very rudimentary prototype can be found from around 3000 BC.
The Egyptians Chairs that came around the 3100 BC might not have been too comfortable but they had looks that could make heads turn and were mostly used by the kings and the royals.