What is the main function of the right side of the heart?
Pumping deoxygenated blood to the lungs.
After blood has been pumped around the body by the left side of the heart, it is brought back to the right side of the heart by the vena cava (technically it is by from the superior and inferior vena cava but they both meet. At this point, the blood is at a very low pressure with little oxygen. The blood is pumped back around the remaining parts of the body and to the lungs by the right atrium and ventricle. It then travels up to the lungs by the pulmonary artery.
In short, the blood would not have enough pressure to travel the remaining distance without that second pump, which is why we have two sides of our hearts. It acts like a petrol stop where you can quickly recharge before finishing the journey.
A quick thing to remember is that the right side has less cardiac muscle than the left as it doesn't have to pump blood as far (it's just topping up the pressure to last until the next big pump).
Hope this helps and let me know if I can do anything else:)