They didn't actually start as raised. A couple of years back it was just a level area of soil divided into four by two crossing paths to give what is a guess a fairly common arrangement for a four-bed rotation. Since I decided not to dig any more I've changed to eight long beds four feet wide and added about 6" of compost in total over two winters, whereas the paths have just had a bit of woodchip shovelled on to help suppress the weeds and cover the soil so the paths don't get too muddy when it's wet. The beds are now definitely starting to look raised as a result. What with blackbirds and other creatures that like to come in and have a bit of a dig about, the division between the beds and the paths has got a little blurred though.
Particularly where they're making new beds on top of existing vegetation, some people start with wooden-sided raised beds and remove the sides after a year. I couldn't justify the cost of the wood and sides often provide lots of places for slugs and snails to hide, so I was happy enough to do without them from the start.
James