For velveting:
This’s used for basically the same reason you’d brine in Western cooking.
Flavor, mostly – to balance the salt.
Cornstarch’s really important here. The thin layer of starch does two things: first, it slows down the heat transfer – as the meat’s cooking, the starches are gelatinizing, absorbing some of the energy. Second, the gelatinized starch helps also makes a moisture-resistant barrier, trapping juices inside the meat.
Ok, so I know that alcohol in marinades is a pretty contentious topic, but for this kind of marinade, it’s a must. My hypothesis is that the alcohol changes the texture of the cornstarch coating and promotes browning, but I’ve never tested it myself… Chinese sources say that it’s to remove the (gaminess) from the meat, so we’ll just go with that.
To season the meat. You could either use light soy sauce or dark soy sauce, depending on the effect you’re after. Dark soy sauce gives the meat a deeper color. Sometimes you want that, sometimes you don’t.
To coat the meat after mixing everything in. Helps form a barrier so that things don’t dry out when marinating.