Why blood circulate?

Answer 1

The red blood cells are transporting oxygen and carbon dioxide throughout the body. They need to move constantly in order to reach the extremities, since pretty much every part of your body requires a constant flow of oxygen. Once deoxygenated, the blood cells must return to the heart to get pumped up to the lungs, continuing the CO2 & O2 exchange.

Its not just for your RBCs though. Your white blood cells also need to reach sites of infection. The circulation is in part to keep everything clean as well. Think about how running water (a river) is always cleaner and safer to drink than still-water (a lake). Because everythings being flushed out. Same with your body.

All in all, it circulates to keep things moving and working. (:

Answer 2

It's a transport system - for :

gasses, "food" (molecules needed for energy and growth), information (hormones), heat, protection (white cells), and pH control.