One of the nice advantages is that they can be used to grow plants that you want to contain in a small area, because they are invasive and will take over your yard. Whiskey barrels are perfect for things like mint, for example.
Another nice advantage of these is that they put the planting about 2 to 3 feet off the ground, so there is less bending over if you have back problems. You can easily keep up with the weeding without all the back pain. I also like the fact that you can grow grass right up to it, and you can run your weed wacker right up next to it and never damage it.
The extra height can be an advantage for vegetables too. It keeps them off the ground high enough to deter some garden eaters, like rabbits. It won't deter the raccoon or squirrels, but you can also cage them too. Another nice thing it they can offer a way to garden for those who really do not have a place to grow things. Just set them on your porch, and fill them with soil, and plant what you want.
Anyway, I thought I would share some photos of how I use them in my landscaping. The the barrels in the photo above are the ones I planted Nasturtiums in this year, and one below I have chives, onions and more nasturtiums. My wife and I like Nasturtiums for salads, and they also are not bothered by garden pests, so they endure all summer long. Happy gardening!