To teach us how to deal with others, and with how the world goes. Those who can't read or write, for instance, are often at a disadvantage in a world where other people can. Education also prepares people to earn a living, which is useful.
to make us all gain a good knowledge about all different aspects of the world through the different subjects and then if we choose specialise in one and maybe make important discoveries for mankind. If we didn't have education i doubt we humans would have come so far, so fast.
Ideally so that we become more rounded people thus connecting better with others and the world around us, and perhaps contributing something during our brief span. Does anyone want to be a savage for the odd half century plus? Out of the things to know about there is something for everyone! The first "building bricks" of knowledge can bring a person on to the next...or, education can be viewed as a key which unlocks the other doors. Unfortunately it has been reduced to gearing people up to join the treadmill, so that there will be a new generation of good little tax-payers. Without the right bits of paper anyone can be classed as "unqualified" even though they might be educated.