Sugar can be added as an ingredient to sweet and fruit doughs for flavour. It can also come from the breakdown of complex sugars such as starch and cellulose that occur naturally in the flour. These complex sugars are broken down by yeast enzymes and become fermentable sugars, which act as food for the yeast.
The sugar contributes to the crust colour during baking in the caramelisation process. It also acts as a crumb brightener and softener and helps preserve the bread.
Sugar levels will also affect rate of fermentation by the yeast, performing best at 5%, being retarded at above 10% or below 5%. This needs to be taken into consideration when adding sugar or fruits to the dough.