There are many different types of vitamin D synthesized in our bodies which require sunlight to form. The many varieties of D are used in an enormous amount of different chemical processes in our bodies- many of which are quite complex and not fully understood. Vitamin D supplements consist of (mostly) one type of Vitamin D. People are typically deficient in that specific type so mild supplementation is a reasonable…
That said you're probably better off getting it from catfish or salmon (which also contain omega fatty acids, a "good" fat which you might otherwise consider supplementing if you aren't getting enough in your diet)
And of course it's not a dichotomy, you can supplement D3 daily and also try to eat a lot of fish!