What is a cask siphon used for?

A cask siphon is specifically designed to facilitate the serving of cask-conditioned beer. When a cask is upright, it allows for the natural carbonation and flavors developed during conditioning to be preserved. The siphon allows for beer to be drawn from this upright cask, ensuring that it is dispensed properly without introducing excessive oxygen. This is important in maintaining the quality and characteristics of cask beer, which is typically served without additional carbonation.

In contrast, the other options describe processes that do not align with the primary function of a cask siphon. For instance, carbonating beer or filtering impurities involves different methods or equipment altogether, and siphoning from a standard keg would not require the specialized design of a cask siphon tailored for cask service. Therefore, the chosen answer accurately reflects the specific use of the device within the context of cask ale service.