[quote author=richard clues
Those guys must use loads of water doing it that way, I know it don't cost much, but still waste of money and water and charging the batteries all the time if they don't have a split charger on, all for the cost of a £40 basic controller,
[/quote]
Errrmmm..... No. The whole idea is to control the amount of water going to the brush without using an electronic controller. Setting the position of the water bypass valve allows more (or less) water to reach the brush.
The advantage is that the only part that could jeopardise the operation is the pump itself, which is much more robust than the controller or the pressure switch.
Regardless of the pros and cons, the original question was something like: "Is it necessary to use a pump controller?" The simple answer, with examples, is "No".