Actually, there is a safe zone where the motor won't rotate, and all the valves are closed or down enough to prevent bending. The factory cam tools use the crank holding tool to hold the engine at this safe zone, which is near, but not at, TDC compression.
If the valve covers are off, and the cam gears need moving around to get them back to the right position, it might be easier to remove all the rocker arms. This closes all valves, prevents any valve bending possibility, and allows for easy setup of the chains and gears. Then pop them all back on when finished.
Otherwise, the chains have to be on as the motor is turned, or the pistons will be moving while the valves stay the same. If you put the motor at the safe zone before you take the chains off, it resolves the issue of how to rotate the engine without some pistons hitting some valves.