The main purpose for crop-rotation is to prevent the build-up of pests or disease in one area of the soil. By changing the host plant, the diseases can't flourish year over year. Really, it's best to rotate families, for example, tomatoes, peppers, eggplants, and potatoes are all of the same family.
Now, if you have a smaller garden, it may be near impossible for you to not plant any plants of a family where plants of that family grew the year before. However, it would probably be sufficient to at least rotate varieties.
But, as RBG said, that all has to do with annuals, or at least what grow as annuals more or less (tomatoes, for example, are really perennials when grown in the right conditions).
Perennials, especially herbs, tend not to suffer from disease build-up like other crops do.
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