Q: Why do some recipes state a specific spoon, like a wooden spoon, to mix the ingredients?
A: Different spoons are used for different mixing methods and it really depends on the type of recipe you are making and the ingredients that are being combined. A wooden spoon is generally used in those recipes that require vigorous mixing and stirring, especially thick, sturdy or heavy mixtures. These mixtures are the ones that are quite robust and don't mind some strong mixing. Also, because their consistency is usually relatively thick a metal spoon or spatula won’t be as effective and will make combining the ingredients quite difficult.
However, when a recipe asks you to ‘fold’ (such as when incorporating whisked egg whites into a base mixture or when making muffins) or ‘gently combine’ a combination of ingredients, the best utensil to use is either a large metal spoon or flexible spatula. Their clean ‘slicing’ edge are the most suitable to use to cut through and gently combine mixtures so that as little of the incorporated air is lost and/or the mixture doesn't become ‘heavy’ due to over-mixing.