Why is blood at its highest pressure when it leaves the heart?

Because pressure is built up in the left ventricle of the heart before it is forced out through the aorta in order for blood to be forced around to the body efficiently to the cells where it is needed. Without this high pressure, blood would not be efficiently ejected to the body and it can pool in the ventricles, which can cause problems.

This high pressure in the ventricle is also required in order for blood to travel around the body. This is because blood moves from an area of high pressure to low pressure to travel. This same principle is used for cells to gain oxygen and other necessities. For example, oxygen will diffuse from the capillaries (smallest blood vessels) which has a high concentration of oxygen into cells, which have a low concentration of oxygen.

Hope this helps ! :)