Of course, there’s flavor. Butter adds its own distinctive creaminess, while olive oils add their own notes. When you’re using fat in small quantities, though, it might be hard to tell the difference between a bread made with olive oil or one made with butter. Then again, you can also use infused and flavored oils to easily add flavors to your bread.
Besides flavor, fat affect the texture of the finished bread. The fat coats the gluten strands and makes the finished product more tender – both the crumb and crust – and it makes the crumb more fine grained. It also makes the loaf seem moister. Breads that are made with fat in them don’t dry out as quickly, so the shelf life is improved.
When you add the fat to the dough is also important. You get better gluten development if the fat is added after the gluten has been developed than if you try to knead a dough that has the fat already incorporated.
When fat is added early in the process, it coats the flour and makes it harder for the flour to absorb water. The fat also bonds with parts of the gluten that does form, so the gluten that exists can’t easily bond to other bits of gluten to form long strands. What this means for doughs with just a little fat is that you'll probably need to knead a little longer to develop the gluten.
When bread recipe has a lot of fat in it, the effect is more pronounced. A dough with a lot of fat can take much longer to rise, and that issue is complicated by the fact that many fat-laden breads also have a lot of sugar. The excess sugar also retards the rise.
For brioche-like breads with a lot of butter in them, you can manipulate the texture of the resulting bread by changing when the butter is added. If the fat is added at the beginning, before the dough has been kneaded, the texture will be more cake-like. If the gluten is allowed to develop before the butter is added, the texture will be more bread-like.
While some fat-laden breads can be dense, a small amount of fat – and particularly a solid fat like butter – slightly improves the volume of the bread.
The fat you choose can also affect the color of the bread. Vegetable shortening won’t impart any color at all, while a deeply colored oil has more effect. I’ve used very green grapeseed oils that made the loaf a golden yellow color. Besides adding a little color to the interior of the loaf, butter also helps the crust brown.
Technically, different fats aren’t completely interchangeable in bread recipes. For example, liquid fats are 100 percent fat, while butter contains some water and milk solids along with the fat, and vegetable shortening has quite a bit of air in it.
In practice, though, the small amount of fat in most standard bread recipes means that you can substitute the fat you prefer without the need for major adjustments. There will be subtle differences in the finished bread, but it won’t make it go horribly wrong.