Corned beef is usually made from especially tough cuts such as the brisket and plate, so preparing it in a Crock-Pot or other slow cooker is an obvious strategy. The long, slow cooking time leaves the beef juicy and tender, and unlike stovetop cooking, the slow cooker won't fill your house with billows of strong-smelling steam. All you need to do is add enough liquid to cover the beef, and let your cooker do its work.
Slow Cooker Versus Stovetop
One of the difficulties of cooking corned beef on the stovetop is maintaining the pot at a gentle simmer as water evaporates. As the level of water drops, its temperature rises to a boil, which makes the already-tough muscle proteins in the corned beef shrink, contract and toughen further. That's not an issue in your slow cooker, which raises its temperature to a simmer over several hours. The lid stays on to trap moisture in the cooker, so the water level doesn't change over time. As long as you've started with enough liquid, you'll have enough eight to 10 hours later when the beef is cooked.
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How Much Liquid
Most cookbooks advise that if you're adapting a recipe for use in a Crock-Pot or other slow cooker, you should reduce the amount of liquid ingredients. That's partially because the slow cooker retains its liquids, rather than letting them evaporate. It's also because the meats you're cooking will release their own juices during the long cook time, which will also collect in the stoneware crock rather than evaporating. This rule thumb doesn't apply to corned beef, in part because cured and salted meats are better at retaining their juices. There are also practical reasons for using enough liquid to submerge the beef.
You might not have thought about it consciously, but you already know that air is a relatively poor conductor of heat. You can set your oven at 200 degrees Fahrenheit to keep your plates warm, and reach into it barehanded without discomfort. Water at that same 200 F will scald your hand painfully, because liquids are more efficient conductors of heat than air. If your corned beef is submerged in water, beer, cider or another liquid, it will cook more quickly than it would in dry heat. More importantly, the whole cut cooks evenly. Otherwise, you'd have tender beef below the water line and tough beef above it.
Submerging your corned beef completely has a second purpose. Unlike bacon and most hams, which are salted only enough to create their familiar and pleasant flavors, corned beef is not intended to be eaten "as is." It's salted to a much greater extent, to produce a more durable product, and is unpalatable unless the salt is simmered out. This requires immersion in water or another cooking liquid, and some might even require pre-soaking before they're cooked. If your beef wasn't completely submerged, the portion outside the water would retain its salt and be essentially inedible.