Although these sweeteners are not essential, they can make the bread more tender because they postpone protein coagulation and thus allow the dough or batter to swell to a greater volume before heat stabilizes the structure. They can also add flavor, deepen the color of the crust, and extend the bread's shelf life.
On the negative side, they add calories and, if too much is used, their caramelization can discolor the bread. A superabundance of these sweeteners can also slow down the yeast's growth enough to keep the dough from rising properly. When substituting molasses or honey for sugar, remember that your dough will require a little extra flour to keep the optimal balance between the dry and liquid ingredients.