This is an important one when making cakes and cookies! It is always a good idea to make sure your eggs and butter and other cold ingredients are at room temperature when you begin because this will optimize the amount of air incorporated into your mixture, and allow for even mixing of the ingredients! Most of the time, you’re asked to cream butter and sugar together, and when your butter is cold and firm, it’s not going to coat the sugar crystals the way it should. It may actually crush the sugar crystals, which have a somewhat star shape, crucial in catching air!
The only time this rule doesn’t apply is when you’re making pastries, like scones, croissants, tart shells, puff pastry.