If something is outside during the day then that something will receive sunlight.
Another source of vitamin d is sunlight.
Being outside is a good way to get vitamin d.
What is a health benefit you receive for being outside?

(a) Relieve pain
(b) staying warm
(c) exercise
(d) inactive
(e) vitamin D
(f) air cools
(g) layers of fat
(h) sweating