This isnt entirely correct, yes it is fat soluble, however that doesn't mean 100% of what you are ingesting is being stored. The 100% daily value varies quite a lot, also if you're deficient you want to be taking more than the daily value. Also it's not universally agreed exactly what the upper and optimum levels of Vit D are, different countries have different guidelines for upper, lower and optimal.

It's difficult to overdose of Vit D, you would have to take a LOT.

Vit D is very important for bone health, and immune function.