When the cleaner is in the upright poistion the microswitch turns OFF the brush motor. If this didn't happen the brush would continue to rotate in one place on the carpet and could cause wear, it would be a bit of a bind to have to remember to do it each time.
The switch on the handle, near the main switch, allows you to turn off the brush motor when you don't want it to run, i.e. on a shag pile carpet or other delicate items.
On my DC24 at odd time the head brush will not start immediately after using the suction tools or will run for a few minutes then stop. The Dyson tech changed the handle switch but it hasn't made any difference.