If you don't allow your bread dough to rise, you'll end up with a heavy and disagreeable loaf.

When you let dough rise, you're giving yeast a chance to leaven the dough. This happens when fermentation produces carbon dioxide gas that puffs it up. Dough should be left to rise in a warm place without draughts and covered with a clean kitchen towel to prevent a crust from forming.

The formal name for this process is proofing (sometimes calling proving).