People often talk about the need for cofactors for vitamin D such as magnesium, boron, calcium, vitamin K-2 and zinc. These work together to help calcium get to where it should be, but they do more than just that.
The link below takes you to a very brief article which explains how magnesium works with your vitamin D stores. This is important because it explains how magnesium is a major part of the process to convert your 25 (OH)d into the active form of vitamin D which is responsible for many of the health benefits mentioned in the multiple links in the article above.
Magnesium itself has many health benefits, but together with vitamin D, they are a formidable pair! The article also gives an interesting list of symptoms associated with low magnesium levels.