Vitamin D3 is sometimes called the vitamin of the decade. Never was there so much new research on a single type of vitamin in such a short time.
Literally thousands of publications came out with new aspects every time in various areas of health. In brief; the thought reference values in blood tests are far too low, the RDA (recommended daily allowance) is so low that it has hardly any effect and just about the entire population is deficient.
The only exception is the group of sun worshipers in the summer, but they too have large deficits in the winter. In Europe, you cannot build up vitamin D3 at all during the winter due to a lack of UVB light in the sun. Supplementation is therefore highly advisable.
And if you do not regularly come outside in the middle of the day in the summer, use during that period is also highly advisable.