If salt dehydrates the meat, then why would brining make it more juicy as a whole?
Actually, it's a popular misconception that brining works because of osmosis. If it was really osmosis at work, plain water would work better than salted water. Kenji over at The Food Lab went into this a few months ago: http://www.seriouseats.com/2012/11/the-food-lab-the-truth-about-brining-turkey-thanksgiving.html
Here's the relevant bit:

To understand what's really happening, you have to look at the structure of turkey muscles. Muscles are made up of long, bundled fibers, each one housed in a tough protein sheath. As the turkey heats, the proteins that make up this sheath will contract. Just like a squeezing a tube of toothpaste, this causes juices to be forced out of the bird. Heat them to much above 150°F or so, and you end up with dry, stringy meat.
Salt helps mitigate this shrinkage by dissolving some of the muscle proteins (mainly myosin). The muscle fibers loosen up, allowing them to absorb more moisture, and more importantly, they don't contract as much when they cook, making sure that more of that moisture stays in-place as the turkey cooks.

In the article he discusses the downsides (flavor dilution) and alternatives to brining (salting). I'd check it out, it's a good read.