What is the purpose of sugar in baking plain bread?
Sugar has a few effects in bread:


It helps make it soft and tender by absorbing some of the water and slowing down the formation of gluten strands.
It feeds the yeast, resulting in a faster rise.
Via caramelisation, it aids in the browning of the crust.
It acts as a preservative, keeping the bread fresher for longer (though 1tbsp probably doesn't make a lot of difference).
It does have some effect on the flavour.