Wrong, wrong, and more wrong.
You do not need to turn rotors if they are not warped or have excessive wear prior to changing pads.
Your pads will not wear faster, more unevenly, or be less efficient.
Turning (or machining or cutting) rotors is not ALWAYS a good practice. If you don't need this service, it should not be done. The thinner your rotors get, the more likely they will be to warp.
The machining process removes iron from one or both sides of the rotor to make both sides perfectly parallel with each other. The more metal you remove, the less is left behind to dissipate the heat caused by braking.
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Guy , You are absolutely right !!!