Soft Pegs (Also known as Spiles.)
Soft pegs are used to vent the cask and release Co2 as part of the secondary fermentation or conditioning of cask ale. Once conditioning has settled down they are replaced with a hard peg to seal the cask, remove the hard peg when serving & replace with a fresh clean peg.
Pegging and venting will bring the beer to its ideal condition. It also helps with the clarification process and, once the beer is on tap, it will help maintain the right amount of carbon dioxide in the beer.