Adding fat to a dough makes is soft and fluffy and generally gives it a gentler mouth feel.
That does not mean that it is better, but it has its place. I personally love enriched dough.
There are certain things to consider when adding fat. Mainly the amount of fat that is being added.
A little splash of oil or a spoon of butter can be added at the beginning of mixing. Whilst a large amount of fat should be added after some gluten development.
Gluten is made from two proteins working together (glutenin and gliadin). As soon as you add water to flour and hydrate it, they start forming gluten.
Adding fat to flour will coat it and hinder the proteins from being connected. That does not mean that it would be impossible. It means that you would have to knead your dough for longer if you added a lot of fat at the beginning.
Longer kneading time means higher temperature. And we all know what happens when the dough is too warm.
Watch the video to see a side-by-side comparison.
Watch the video here