Once the ingredients are combined, it is important to build a strong gluten structure to capture the gases that the yeast creates. If your dough does not have a strong gluten structure, the gases will simply release out of the dough, instead of being trapped inside.
Gluten will form in a dough naturally, given enough time. This is why no-knead bread recipes work, because time is the element building strength in the dough. However, kneading the dough is the other main way that gluten structure is built in yeast bread. This method builds a stronger gluten structure quicker, which allows breads to be made with more yeast in the dough and shorter rising times.