To better understand why it is used imagine a map, going right is +, going left is -, going up is i, going down is -i. Turning left is multiplying by i, turning right is multiplying by -i.
So i is used to calculate things where you need 2 dimensions.
To better understand why it is used imagine a map, going right is +, going left is -, going up is i, going down is -i. Turning left is multiplying by i, turning right is multiplying by -i. So i is used to calculate things where you need 2 dimensions.