According to various people who own them and confirmed by a few books and internet articles I've read, a raised bed offers a number of advantages over simply digging a hole and planting vegetables:
There were a number of reasons why I decided to design and build one myself (as many people do, it's trivial).
The 'headboard' and its posts provide anchor points for 50lb fishing line to be strung across in a grid, much like a soccer net that in turn gives climbing plants needed support. It isn't expensive and doesn't rot or stretch over time (at least, not within the time frame of a growing season), and it's almost invisible. Some details I should point out:
The planter is 8ft by 4ft with varying depth. 30 bags of miracle grow mixed with 10 bags of black soil later, and it's full.
A jiffy grow kit, some seeds, and a paper map of which seedlings are which keeps everything organized until they are big enough to transplant. Depending on the plant, some seeds skip this step and just go straight into the soil.
A few weeks after planting, things are growing well.
Looking quite decent after about six weeks.
Towards the end of summer, returning from a two-week vacation, stuff had got way out of control. I did learn some things in the process: