Commercial horticulture is market driven and therefore crops are often grown in places that they wouldn't naturally occupy. This leads to increased use of chemical fertilisers and sprays, as plants, like people and animals, are not happy if they are living in places that don't suit them.
In organic growing we try to grow plants that work without too much interference. The idea is that when you start a new garden, you'll find that some plants do really well and others poorly, and often not the ones that we would have thought. By only replanting crops that do well naturally, we can avoid too much interference. Be prepared to give up on some of your favourite plants if they don't do well- otherwise you'll forever be battling diseases. For example, as a wine drinker, I would love to grow my own wine grapes, however I've had to give up on them as there is too much moisture in our soil and they get too much disease- it simply isn't worth adding volumes of copper and sulphur to my soil just so I can grow grapes. The bonus is that the Granny Smith tree loves our place, so we get a fantastic apple cider every year!
There are two main reasons why you should rotate annual crops. Firstly the crop will deplete the area of the nutrients that it needs the most, and it may even add nutrients that it doesn't need. For example legumes such as peas will actually increase the available nitrogen in the soil, so if you follow legumes with a green leafy crop such as lettuce it will benefit from the nitrogen naturally added by the peas.
Secondly, diseases tend to target specific types of plants, so by changing the crop type you will discourage the build up of disease in the soil.
Crop rotation is usually worked out by dividing plants into three categories- root crops, legumes and leafy vegetables. For example, plant peas in spring, followed by potatoes, then lettuce. You can play around with the order and the timing, the idea is to keep things turning over by alternating those three broad categories.