"A man without a goal 

is like a ship without a rudder"

Thomas Carlyle

If you dont set a goal to yourself you will go in any direction that doesn't take you where you wanna go like a ship without a rudder. If you set a goal you will drive yourself to that goal and accomplish is like the ship arrives at the destination. For example if you put a goal to have good grades you will work hard for it and if you dont your grades would be all over the place