I guess you could learn, if you call seeing how to make a person's particular sig.
Emulate many, and become individual, but emulation is done by looking at the signature, copying is acheived by seeing all the layers.
LAWL. Looking at PSDs help to show you what different combinations of adjustments do, what other ways there are to do lighting, how to organise your PSDs effectively, and how to achieve different effects. By your logic, your actually saying tutorials are useless as well, as tutorials are Layer by Layer examinations.