Why do some recipes recommend a short rest after mixing but before kneading the bread?
Besides the initial gluten relaxing that mrwienerdog wrote about, another important effect is the absorption of liquid.

It takes time for the flour to fully hydrate with the available liquid. Before it does the dough will be more sticky and there will be less gluten available for kneading. Giving the recently incorporated dough a short rest allows the flour to pull in all the liquid it can and become much easier to work with.

A similar effect can be seen with pastry doughs. Even though often the goal is to minimize gluten development, a short rest gives the flour time to hydrate and can turn pastry dough from difficult to work and crumbly to soft and smooth.