bread making remove butter
Fat softens bread because it interferes with the gluten formation. It also gives the bread a richer flavor.

Many breads, including standard sandwich bread, include fat for these reasons. Butter has great flavor but any fat will do. You should soften any solid fats.

You can probably leave the fat out if there isn't much in the recipe. Expect the bread to be a little less flavorful and a little less soft.  

If there is a lot of fat that is being left out you might have to increase the liquid to compensate for the dough being stiffer. At that point, unless you enjoy experimenting, I would just find a recipe that doesn't use fat.