Well, Remus went into the "practical" part of it. In "theory", a PR (and use this for "photo realisctic" rendering since a straight SU oputput is "rendering" as well - just not PR) so a PR renderer "shoots" photons that bounce from the surfaces and according to the capabilities of the rendering engine, different "further" features (such as reflection, refraction, bumpiness etc.) can be assigned to the materials so they look more realistic. Also, there are more light sources than rthe sole Sun (hm... could even be a good pun) and these lights all contribute to the final render by "shooting" those photons.
Without going into more details (like bounced photons bounce further or not and such), this is the basis of a PR renderer.
On the contrary, SU doesn't do all this. There is the sun (or no sun) and there is the sunshine vs. shadow. On top of all, an annoying "shadow bug" contributes to this limited choice (i.e. if you have sun/shadows on, there are problems sometimes).
Still don't underestimate a native SU output because it can be really cool and combining it with some post-process work, really stunning results can be achieved. It's juswt a matter of "fashion" which taste; PR or NPR (i.e. "non" PR) is more popular at a time.