Why do some ingredients in baking recipes need to be at room temperature?
The exact reason depends on the particular ingredient and recipe.  Some common ones include:


General temperature.  By having all of the ingredients at room temperature, the time it takes to bake the overall item(s) is easier to predict, leading to better, more consistent outcomes.
Egg whites  Egg whites form the protein network that creates the foam when whipped more easily at room temperature than they do cold, so will whip up more quickly and with more volume.
Butter   Butter is often desired slightly cooler than room temperature, for its plastic properties; it incorporates air for the creaming method best slightly below most room temperatures.  Otherwise, it is easier to spread or incorporate when warmer.
Cream cheese  Easier to mix with sugar or other ingredients at room temperature than when cold.