The theory behind crop rotation is simple enough, different crops require individual growing conditions particularly in relation to nutrients. Growing the same vegetables in the same place will eventually diminish those nutrients and reduce the quality and quantity of the crop. Moving the crops around will allow the soil to replenish its resources.
In addition, the spores that cause diseases such as clubroot have a greater chance of building up in soil where the same crop is grown year after year. There's no definite guarantee that your vegetables will remain completely disease free but rotating plots will reduce the risk, especially in a system that is designed to encourage the cultivation of robust plants.
Another advantage of crop rotation is that you can make economic use of fertilizer. Some crops such as brassicas are much more demanding than others, so applying expensive fertilizer and manure to their plot is worthwhile but this is much less important for a bed where for example less demanding salad leaves are to be grown.
As a bonus, some crops such as broad and other types of legume will fix nitrogen in the soil, so your rotation should be arranged to take advantage of this by planting nitrogen hungry sprouts, cabbages and other brassicas in that plot during the following year.
Although you can rotate over a longer period, the most popular rotation timetable follows a three year cycle, with vegetables belonging to the same family spending one year on each plot which has been specifically prepared for them. The exceptions to this rule are perennials such as rhubarb and asparagus which can remain in the same position.
Plot A: First Year: Legumes including all types of beans and peas, plus sweetcorn and okraSecond Year: Brassicas, for example Broccoli, Cabbages, Cauliflower, Kale, Swede, SproutsThird Year: Beetroot, Carrots, Parsnips, Potatoes, other root vegetables, onions and tomatoes
Plot B: First Year: Brassicas, for example Broccoli, Cabbages, Cauliflower, Kale, Swede, SproutsSecond Year: Beetroot, Carrots, Parsnips, Potatoes, other root vegetables, onions and tomatoesThird Year: Legumes including all types of beans and peas, plus sweetcorn and okra
Plot C: First Year: Beetroot, Carrots, Parsnips, Potatoes, other root vegetables, onions and tomatoesSecond Year: Legumes including all types of beans and peas, plus sweetcorn and okraThird Year: Brassicas, for example Broccoli, Cabbages, Cauliflower, Kale, Swede, Sprouts