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I found myself asking this same question while doing the flywheel on my 986 recently. I did a little reading and found that the idea is to get greater clamping force from a smaller fastener. If my understanding is correct, tightening to "torque" puts the fastener into the elastic portion of the strain curve. Tightening to "yield" applies more clamping force, but we are now into the plastic zone of the yield curve.
In the elastic zone, the fastener will return to it's original shape. Once you cross the line into the plastic zone, the fastener is permanently deformed, and cannot be reused. The key, of course, is not to go so far that we hit the failure point.
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