It has no actual use on your heater. However the benefits are;
you can heat the water in the tank- or raise it's temp if it's very cold.
You can get the flow going without anything coming out of the brush. This helps to diagnose and check problems, say for instance you are working on a house and the water stops, or the water goes cold. You would turn the tap off and walk back to the van to see why wouldn't you?
Well with a bypass, when you return to the van you can fiddle about with things such as varistream settings or check to see if auto ignite is working without useing the brush head tap, or continually walking back to it.
I take it I don't have to explain the extra in line filter.