Egg yolks contain fat and cholesterol, while egg whites are fat-free. If the goal is to reduce fat and cholesterol, use egg whites in place of an equal weight of whole eggs in doughs and batters when the egg is used as a binder.
When egg foams are used for leavening, egg-white foams can often be substituted for whole-egg foams. Of course when the eggs are also a main structural component of a baked item, using egg-white foams in place of whole eggs makes too great a change in the product. For example, if you substitute egg whites in a genoise sponge cake formula, the product will no longer be a genoise but something like an angel food cake.