Vegetable gardening is becoming more and more popular these days as people want more control over their food and where it comes from, as well as the desire to save money by growing more of your own food. Key to any garden plot is the soil, if you do nothing else, providing good soil will give you a good garden. Improving existing soil to be good enough for a garden though is rough. If you have a section of your yard you would like to be a garden, to turn that turf into garden would require significant labor, and even some machinery. All in all, it just isn’t the best way to do things. Most home gardeners with relatively small lots instead build raised beds. Raised beds offer a variety of benefits, they’re easy because you don’t improve the existing soil, you merely add soil on top. They warm up faster in the spring, they offer excellent drainage, and the quality of the soil you can add to them far exceeds any result you would get by trying to till up your existing soil. You can build a raised bed out of almost anything, an old tire, stone, brick, plastic, but the cheapest option tends to be wood (unless you have a free supply of old tractor tires). Since cost of food is a large motivator for someone to build their own vegetable garden, wood ends up typically being the material for choice for non-ornamental (ie, edible) raised bed gardening.
Source: Backyard Gardening