it's based on a visual novel, so they go through the various routes.
Passing on the original and the light novels is fine. But don't skip Fate/Zero! It is a necessary prequel that colors in a ton of backstory. I'd say it is nearly required watching for Fate/Stay Night: Unlimited Blade Works.
it's a shame they skipped chapter 17... not because it introduces Polt. (STOP TAUNTING US!)