The typical failure modes mean that it'll be marginal for a while, and should be able to give you information before it goes fully bad. Being able to have introspection before it goes fully bad is what I want, as I hope to have some data before it even makes it's way on to a bench.
The power clients (mainboard, GPUs, disk drives, etc) normally know they're getting pretty bad power from a PSU and filter it heavily to smooth it out. It'd be nice to know if the voltage is occasionally dropping and it's just being covered up by downstream caps that are over provisioned to just smooth out not fully rectified AC.