It's not necessary, but it's convenient. What's actually happening here is Monte Carlo integration. Randomness guarantees you get the right answer. It prevents correlation artifacts. It also gives you a nice film grain look when you don't have the time to take samples until complete convergence.

you get unwanted artefacts in the results if you don't use some randomness. having said that, there might ways of reducing the need for it (which would be more efficient).