Because you mention planting "on top," I assume you mean on a raised area between furrows, which are technically the low trenches you get from turning soil to the side. Most of the vegetables you mention will do just fine on level ground if you have good soil. But in many cases, especially if your soil is a heavy clay, a raised area may work much better because it will drain easier. A waterlogged soil can be detrimental to most, if not all, crops. There are other advantages to planting on raised soil, too. In colder climates with shorter growing seasons, raised beds offer the advantage of warming up earlier in the spring. Also, by walking beside the raised areas it is easier to avoid compacting the soil.

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