Traditional whiskey sours (or any other "sour"), among other classic cocktails, are made using an egg white. When shaken well, it creates a foamy layer on top of the drink that is quite stable and offers a pleasant texture (IMO), somewhat similar to the foam you get on top of a freshly poured beer. On top of that, there is some sort of chemical reaction that happens with the egg and alcohol, especially oak-aged spirits—apparently the egg white "softens" the booze, which is nice for higher-proof whiskeys and such.[0]
You can take this even further, too: I make a variation of the Mai Tai that's topped with a foam made by blending egg white with passion fruit puree. The foam is the first thing you taste, so you immediately get hit with the sharp, sweet-tart flavor of the passion fruit, before the rest of the (more subtle) flavors kick in. It's delicious.
(I've done it with aquafaba and it works almost as well, although I've only used canned, which generally contains salt. I don't mind it, but it doesn't work in every cocktail.)
[0]: If you're interested in this stuff, I highly recommend the book Liquid Intelligence by Dave Arnold.
You can take this even further, too: I make a variation of the Mai Tai that's topped with a foam made by blending egg white with passion fruit puree. The foam is the first thing you taste, so you immediately get hit with the sharp, sweet-tart flavor of the passion fruit, before the rest of the (more subtle) flavors kick in. It's delicious.
(I've done it with aquafaba and it works almost as well, although I've only used canned, which generally contains salt. I don't mind it, but it doesn't work in every cocktail.)
[0]: If you're interested in this stuff, I highly recommend the book Liquid Intelligence by Dave Arnold.