On a very small plot, crop rotation may be different from what you do on a large plot.
Hypothetically, let's say you only have a 4 meter × 4 meter plot. You grew tomatoes on half of that, French beans on the rest of it. Scenario 1: everything grew normally, no apparent diseases. Scenario 2: the tomatoes had blight, the beans had rust.
You might be able to get away with no rotation in Scenario 1.
Scenario 2, rotation becomes more important, you would be better off growing something else for a couple years (get tomatoes and beans from somewhere else). Hypothetically, courgettes and corn one year, something else the next year.