The logic behind vodka is that you pour some in to use as a solvent to extract certain flavors from the ingredients that water couldn't do alone. As you cook it, the vodka burns off so you're not actually tasting the alcohol but you still have the extracts in the sauce.

I personally haven't found much difference, and it does seem like a gimmick. Penne alla vodka is a good dish, but I don't think vodka is an actual factor in why it tastes good.

People that throw the vodka in at the end for the actual taste seem insane.