You left out a couple of things here. One is that ale can also be served from keg, and commonly is, so the cask version is often known as "real ale". Guinness, for example, is a keg ale that uses nitrogen to mimic the mouthfeel of real ale.

The main thing, though, is that cask ale is easily the best form of beer that there is, when it's not off. Something about the sparkling freshness of beer that just finished fermenting in the cask, the cellar-cool temperature, and the low CO2 just makes for an incredible taste experience. Most beer goes downhill from the moment it leaves the brewery, but cask ale is at the very height of perfection the moment the cellarman puts it on. Well-kept cask ale is simply the best thing there is.

Interestingly, the best way to drink lager is in Bavaria, from wooden barrels, with low CO2 and relatively warm. It's not quite as sophisticated as real ale, because there's no life in the barrels, but it's still great.