There is something significant about learning to coordinate the firing of signals to a dozen muscles at once (and maintain those GO signals for the duration of the rep) which cannot be replicated any way other than lifting a big weight with one or more major muscle groups.

Also, as an aside, for the duration of that one rep, (if the weight is sufficiently heavy for the lifter) the mind narrows focus completely - no other conscious thoughts are present at that time. This is an interesting contrast to how many of our HN-type minds usually operate.