There's a lot of data to suggest the "normal range" is still way too low. So, you may be getting the ok on your blood test but still be suffering from the effects of a lower level of vitamin D. Additionally, places that get a lot of cloud cover in different seasons may cause your D level to drop quite a bit in the Winter.... which means you need to test often. Also, your skin color is going to make a difference in how much D you absorb. If you live in a cloudy northern place and are darker skinned you may not get enough even with sun exposure.
Anecdotally, I feel better supplementing in the winter.
Vitamin D is a fat soluble hormone. During much of recent human evolution, the typical pattern was for people to accumulate body fat during the spring / summer / autumn seasons when food is more plentiful. Then they would survive partially on those fat stores during the winter when food was scarce. Breaking down adipose tissue for energy also releases stored vitamin D (and some other nutrients) into the rest of the body. So, they maintained adequate levels even with minimal sunlight exposure.
Now in developed countries we have screwed up that pattern and no longer survive on stored body fat during the winter. If anything we're more likely to gain fat during the winter, which makes vitamin D levels even lower. But we're no longer at risk of starving to death in a hard winter, so that's a positive.
Anecdotally, I feel better supplementing in the winter.