We all know that calcium is important in bone health... but it's not as simple as that. Firstly, as this infographic shows, calcium utilization by the body actually requires more than increasing the calcium intake!
What isn't shown here is that bone density is also determined by the mechanical forces (slight bending, twisting etc) placed on them through day to day activities. This makes sense: If you are frequently lifting heavy things you need the skeleton to handle the strain that your (often larger) muscles put on bone in order to move the weight. The heavier you go, the stronger your bones need to be.
So when you next visit your box in search of that elusive deadlift PR, think to yourself: I'm working out my bones too!
Here's a reference for further reading: http://www.medscape.com/viewarticle/736929_2