title_body: Why is some beer kept in/served from a firkin in a bar?
upvoted_answer: Yes the same beer from a cask and a keg will be VERY different.
  
  To your first point, breweries and bars will often have specially treated casks in ways that are impossible to do with kegs.  Stuff like dry-hopping the beer in the cask at the bar so it's at its freshest and most intense.  With a keg you'd need to put this between the tap and the glass like a Randall but with a cask you can just pop the cap and shove whatever you want in there.
  
  Casked beer also generally still have active yeast in it, at least according to CAMRA's rules, while kegs are filtered and have no yeast in them.  Because of this there's a lot of preparation involved in a proper real ale cask.
  
  
  The cask will come sealed from the brewery.  First a soft spile or peg must be driven into the top hole.  This is just a porous plug that allows air out.  Based on how foamy this gets the bar can see how active the yeast is.
  When the yeast dies down the soft spile gets replaced with a hard spile which stops gas exchange.  The cask then has to sit for a while so all the yeast can fall to the bottom as well as any other particulates or large proteins in the beer.  This is called "Dropping Bright"
  Finally, once the beer has dropped bright the cask can be tapped which involves removing the hard spile and also either hooking up the side hole to a beer engine or a simple tap.
  
  
  Because of all this and traditionally being served from a cellar or bartop at "cellar temperature" vs kegs which are refrigerated the beer that comes out will be slightly warmer (40-50F vs 30-40F), much slower because of the pumping and off-gassing from the pour, and way less carbonated.  Some beer engines will also use "sparkler" taps that fizz up the beer even more on the way out to generate more head but also results in a less carbonated beer.
  
  The result is a beer that will generally be smoother and rounder, you may get more flavor and more sweetness since it's warmer.  Some flavors will come out more in the warmer and less carbonated and some will be overshadowed.  Mouthfeel will be affected a lot by both factors as well.  It's a hugely different animal and sometimes people like it.