I think one way to look at it is this. The peak HP rating doesn't tell you the full story of how much work you can accomplish in a given amount of time with a given engine in a given car, because the engine doesn't spend the whole time at the HP peak. The peak torque number is just another data point that gives you some idea of what the power curve looks like, and thus provides some insight into the "area under the curve" that a given engine will provide in a given car.
in a heavy car with highway gears and a tight converter, the engine will tend to operate at lower RPMs. as a consequence, the data point for the peak torque number is more important than for a lighter car with gear and looser converter that spends less time at lower RPMs.
in either case you are trying to optimize average power.
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