This applet illustrates the addition of two vectors. You can move the points around and then use the sliders to create the sum. Move around the points to test various cases until you are convinced of the property.
Explain how to add vectors graphically in your own words. Extend the concept to include the addition of more than two vectors.
Assume that on the first day of my camping trip, I walked 5 kilometers from the base camp and another 5 kilometers on the second day. How far am I from the base camp? Even if I promised to only move along the x-axis, you can't answer that. It is not enough to state that I walked 5 kilometers. I have to tell you whether I went left or right. In 2 dimensional space, there is an infinite number of possible directions, not just left and right.