That is supposed to teach you how a function behaves. Doing it on a computer doesn't give you the FULL effect that doing it by hand does. You know... Put in a few numbers, get a few numbers out... connect the dots.
They still make you do things by hand in higher math classes. "Plot this ellipse." "Do a surface plot of this equation." "Draw this magnetic field." etc.
Physics and math will have you do that.