For multiple reasons. Achieving the potential peak bone mass during adolescence, and maintaining it during the adulthood is very important for preventing future fractures. There are many diseases, medications, lifestyle habits, and food components can adversely affect the bone density and the bone quality leading to fragility (osteoporotic) fractures. If one takes proactive actions, most of these are preventable.
Bone density. This test is important to detect thinning of the bones. We all start loosing bone as we age and some women are more prone to loose more bone than what their body can restore. Osteoporosis could lead to friability of the bones and at risk for fractures. A hip fracture can lead to severe complications and even death, 50%mortality within 6 month after a hip fracture.