Some plants take nutrients from the soil others contribute nutrients. If you have the same plants in the same spot all the time, you will end up nutrient deficient, and your plants will be unhealthy and susceptible to disease. If you rotate crop families you generally end up with a healthier garden. An "ideal" garden, with all vegie families, will rotate quite easily. If you limit what sort of veggies you have, you may find you need to replace nutrients (so if, for instance, you only grow tomato and lettuce, you a not rotate enough and would need to replace nutrients via vegie foods for the soil etc)