Rotating brushes work from the power of the pressure washer. The fast moving stream of water spins the brush. Brush bristles come in different stiffness for different jobs. Smooth painted surfaces and windows generally clean better with a soft bristle while rougher surfaces like fences or stucco need a stiff bristle.
A rotating brush works great for some surfaces but is useless on others. If you want to clean tour car a rotating brush can be help but it does not take the place of a good old soapy cloth. Doing siding with a rotating brush works well.
To get to siding or windows you can use a longer wand or telescopic wand to get there. A word of warning here... a telescopic wand is great for extending your cleaning range but they can be very difficult to control when extended, especially with a heavy rotating brush on the end. Know your limit and stay within it. Otherwise you will end up breaking the wand or a neighbor's window!