Disolving sugar is eggwhites makes the peeks softer and more viscous, as opposed to stiff and brittle, because disolved sugar absorbs the water in the eggwhites. Whipping eggwhites specifically works because when you whip the eggs, proteins are exposed to air, which denatures the proteins, causing them to break and link together into chains. It's both a chemical and mechanical process.

Sugar cannot be disolved in fat, although butter does contain some water (5..20%), and this means a certain amount of sugar would be disolved in the water (thickening), but primarily creaming is a mechanical process by which you use the sharp edges of the sugar crystals, to cut little holes in the fat and incorporate air as a result.

Creaming is far less effective then whipping eggwhites, so you'd rarely see them used together, as there's deminishing returns when you try to combine multiple leavening agents. Your batter/dough can only hold onto so much air, and so the moment you start mixing things together, it'll have a peek at the point where the batter isn't strong enough to hold the air anymore.

This is why bread can be so fluffy despite being a very stiff dough. The stronger protein bonds can hold onto air more tightly, which allows you to get a similar rise that you'd get out of eggwhites. Creaming on the otherhand is far less effective, which is why it's usually combined with baking powder or baking soda, as the mixture is capable of holding onto more air. Similarly baking powder and baking soda have deminishing returns, where they rise the batter prior to baking a little bit, and then double acting baking powder raises it more while it's baking, eventually tho proteins harden and no more rising can take place.

If you're using yeast and making bread dough, you really do not need a secondairy leavening agent, as yeast is just going to produce far more carbon-dioxide then your dough is capable of holding onto. It's possible to mix eggwhite leavening with creaming, presumably you'd reduce the amount of eggwhites and butter... resulting in a different end texture? Most of the time what you'll see is eggwhites mixed with baking powder, to decrease the amount of mechanical work you need to actually put in. Egg proteins have very weak bonds, so it's not typical to see them mixed with yeast or creamed butter, which would be things like cookies and bread and pound cakes, but I assume a recipe demanding for beaten eggwhites would be somewhere closer to a pound cake.