There was a pub around here that had a very good IPA. Just a strong, solid, well-hopped, tasty beer.
But when they served it on cask, it was otherworldly. All kinds of subtle flavors, malt and hops, that you never noticed when it was cold and heavily carbonated. Smooth, too, easy to drink. Tasted so much fuller and complex and "rounder" without the carbonation masking so much of the flavor.
A dark IPA on cask sounds phenomenal, so many of those flavors just get lost.
Cask ale, all the beer flavors that we love but just magnified by 10x.
Now, if the basic beer was kind of flat and boring, serving it on cask may not have helped. This same pub had some kind of brown ale or porter that was just "meh" on cask (tasted like it was fermented very warm, way too estery even for an English ale). But it was a pretty mediocre beer when cold and fully carbed anyway. Serving on cask isn't going to make a bad beer good, if anything it'll unmask the flaws of a poorly-made beer. Serve something cold and bubbly, you aren't going to notice flaws as readily.
Serving on cask isn't going to make a bad beer good, but it'll make a good beer excellent. IMHO.