In my understanding, it makes sense to overpower a PSU during testing in order to confirm, that it will either operate safely, or safely shut down due to OPP (as it should), rather than burn or explode or whatever
Besides, isn't overpowering possible without the intension to do so? Some malfunction like a short on a PCB (which may have high enough resistance not to trip the SCP) could potentially create abnormal load.. that's what one can read in jonnyguru's faq, at least. If I got that correctly.
Also, I understand that OPP essentially limits the current that the PSU can draw from the wall; this could be helpful in case of abnormally low voltage in the mains, which might make psu draw more current than it is designed to handle at otherwise normal load...
I am not an expert, and may be saying non-sense above; I will happily stand corrected in that case.