We generally store our calcium via our bones. As such, calcium in our blood/serum should not be highly prevalent. But calcium is needed for many physiological activities in the body and that is why there is some presence.
However, high or low levels can demonstrate irregularities of our hormones (eg. PTH [parathyroid], aldosterone) or organs (eg. Kidneys). It can also be elevated or decreased with certain medications and require monitoring. It can help determine need of treatment in osteoporosis. Lastly, it can be messed up with malignancies. Usually abnormal calcium have a cause that requires further work up.