Crop rotation is simply a way to ensure that crops are grown in different section of the garden year on year.
It does require a little organisation (ideally you should plan at least a year in advance), but you could reap massive benefits by letting nature do the hard work for you:
The main reasons for doing this is to avoid diseases specific to that vegetable causing problems after being allowed to build up year after year. A good example of this is clubroot, which attacks many brassicas and can be greatly reduced by changing where you grow them each year.
Vegetables such as potatoes tend to smother out weeds, so they are unable to grow.
It’s a good idea to follow these with a hard-to-weed vegetable, such as onions.