Eggs-To Warm or Not to Warm
Any good pastry chef will tell you that it is better to work with room temperature or even slightly warmed eggs than cold ones when making sweets. We, at the bakery, always make sure to take our eggs out of the refrigerator several hours before using them to achieve this result. This goes for whether we are using whole eggs or separated ones (just whites or yolks), and it goes for recipes where we need to beat air into the eggs (meringues) or in recipes where we do not (muffins).
The idea that warm eggs are better when it comes to baking can be justified in two ways. First, say that you are in a situation where you need to beat air into your eggs, either in a meringue or maybe a sponge cake. When eggs are cold, the protein bonds in the whites are wound tighter, making it more difficult to incorporate air into them. When eggs are let to sit at room temperature, or even warmed slightly, these protein bonds loosen so that it becomes easier to beat air into them. Now some cookbooks say that although it is easier to beat air into warm eggs, it is actually the age of the eggs that are truly important because ultimately, proteins from a fresh egg will hold air better once it is whipped. In general though, I assume everyone is using fairly fresh eggs, and I do feel you get more volume out of the eggs when they are warmed.
But let's say that initial reasoning didn't really float your baking boat, I go to reason number two. Let's say you're making something like muffins or a pound cake where you don't have to beat air into the eggs, but you do want the eggs to more or less incorporate smoothly into your just beaten butter mixture. I've seen this a ton of times where when I don't warm the eggs, mixing eggs into beaten butter and sugar form this curdling mixture. It makes sense because butter is a fat and eggs are a liquid. But, if you do take time to make sure that the eggs are at room temperature or if you warm them slightly over a water bath, you will notice that the eggs incorporate so much better into the mixture. Things mix more homogenously when they are at similar temperatures and so you get a more homogenous batter, which is always important in baking. When you have chunks of butter or pockets of unmixed flour floating around, you will see that in your final product.