Why would someone need open heart surgery?

Multiple reasons. The most common cause of heart disease resulting in cardiac surgery is either blocked coronary arteries or a problem with one of the heart valves.
Many reasons. Coronary artery dz in multiple vessels may need open heart. Valve problems may need replacement or repair by open heart. Congenital heart defects would be another reason for open heart.

Related Questions

Why would a person need open heart surgery?

It's complicated. That's a very open ended question. Heart surgery is typically indicated when the risks of surgery are outweighed by the benefits. Lots of studies have been done for various conditions to determine when it is a good idea. Considerations like symptoms (eg, chest discomfort, trouble breathing), or risk of dying (eg, large aortic aneurysms, type a dissection) are factored into the decision.

If someone has had open heart surgery as a child, would he usually have a shorter lifespan?

It depends. Some problems can be essentially cured with open heart surgery, with little effect on the lifespan or activity. Other serious problems result in a notably shorter life expectancy.
What surgery? It depends on what defect was treated. Some defects can be cured with surgery but others can only be palliated. Either way surveillance is important so follow up with the cardiologist is necessary. Some heart defect do lead to a shorter life span.