I started out taking because I was low. I also can't go too long outside without sunblock which therefore blocks the vitamin D absorption. I was told basically take a 15 min walk without it, but unfortunately I will start to turn red in that time. It is what it is. (plus, very little chance to get it outside for about 5-6 months a year where I am)
I take it every day. Even though my levels are "normal" it has become increasingly known that D is so important for all aspects of our health. I just viewed something about men's prostates! It's so easy to take and then whatever you get outside is a plus.
You can google or search for Vitamin D and prostates to get the info I am talking about.