We all know that calcium is necessary for strong bones. Thanks in particular to milk and other dairy products, we should be able to obtain enough calcium, but this is not the case for everyone. And calcium alone is not enough. To properly absorb calcium in the body you also need sufficient vitamin D.
You build up and break down bone throughout your life. In young people, there is more bone build-up than breakdown, so bones become bigger and/or stronger. After your 45th birthday, however, this reverses and the bone breakdown is greater than the build-up. It is good, especially after this age, to ensure that you get enough calcium and vitamin D.