It was "obvious" that heavy objects fall faster than light ones. Until Galileo actually bothered to test it.
The biomechanics are certainly different. If you're curious: In terms of forces making it to the pedals, this is what's going on when you are standing.
You can't tell that this is going on, based on subjective measures. Human beings are not born with pedal-based power meters implanted in their feet.
"Targeted," no. "Relevant," yes. Several respondents are repeating the flawed, subjective, unproven idea that "you add power to the drivetrain on the upstroke!", and it has become necessary to cite the studies in what appears to be a vain attempt to correct their error.
In addition, the point is that on the upstroke, all you're really doing is lifting your leg. You might do this in a slightly more effective manner with foot retention, because you will have better contact and control. But the action, and thus the muscles used, are essentially the same.
Plus, at no point in this thread have I advocated platforms, especially for pros. As I've said nearly a dozen times already, all I'm saying is that cyclists do not apply power to the drivetrain on the upstroke, even with foot retention.