White bread is also one of the cheapest sources of energy, though depending on the geographical location either maize or rice may be slightly cheaper sources of energy, but which provide much less protein.
Among cereal products, wheat bread has the highest protein content, except for oats, but oats are much more expensive per protein content.
If you eat white bread in preference to any other starchy products, like those based on rice, maize or potatoes, this ensures a higher protein intake per a given calories content, which enables a lower consumption of more expensive sources of proteins, like those based on legumes or on meat.
White bread cannot be the only food that one eats, because other kinds of foods must provide other nutrients, but it can provide at a minimum cost major fractions of the daily intake of proteins and energy.
After experimenting with many different healthy diets, I have settled on a variant that I consider optimal, where wheat bread that I bake myself at home typically provides about a third of my daily protein intake, so its nutritional value is quite important (and the fact that it is very tasty is a welcome bonus).