Modern recipes (as far as I know) for bread includes sugar.  Sugar gives the yeast a food source, which supports it growth and allows yeast to give bread many of its qualities.  It hit me the other day that sugar (produced from sugar cane) is a relatively new commodity.  People did not have this kind of sugar 1000 years ago in Europe, the middle east, etc.  How did they manage to make bread without it?  
I assume, by sugar you mean sucrose. However, yeast actually prefers glucose and maltose, see nutritional requirements of Saccharomyces cerevisiae and also proofing. Luckily, we get glucose and maltose "for free" from the flour, see this article on bread chemistry:     Flour naturally contains both α- and β-amylases, which between them   break down some of the starch in the dough to the fermentable sugars,   maltose and glucose   The short answer is thus, bread does not need additional sucrose, as the maltose and glucose we get out of the flour is already sufficient for the job.  On a sidenote, as other answers mention, basic bread dough does not call for sugar in the recipe. Technically, by adding sugar you get an enriched dough (a dough with additional sugar, syrup, butter, oil, eggs, milk, or cream, etc. is called an enriched dough). In this case, the sugar is added for achieving a specific effect, most typically making the dough sweeter or also more tender see What is the purpose of sugar in baking plain bread? and this article and also this paper. 