Whys bone mass important?

It's a bank account. We stop adding to our bone mass by the end of our late 20's/early 30's. After that it is a slow decrease in bone mass. Bone mass can decrease to the point where you bone breaks under relatively normal loads. This is commonly known as osteoporosis/fragility fractures and can happen in the hip, wrists, and spine. It is important to maintain as much bone mass as possible to avoid those fractures.