D3 is one of the most important and a necessary supplements you need to take. Vitamin D is used in almost every function of the body, especially those functions involving repair and bone development and maintenance. Without enough vitamin D, and you need a lot, the body can not undertake basic functions and these undertakings remain undone until there is enough vit D in the body.
If you live in a northern climate, taking 5000 to 10000 IU/day is commonly needed, especially if you spend the majority of your time inside.