Well ideally bitcoin is a means of investment, but if you really need the money for whatever reason, you have to sell the coins to get some sort of fiat currency back.
People sell their bitcoin in a case to case basis, some sell it to convert it into cash because they will use it beyond bitcoin coverage. In some cases, they sell it because their country is trying to ban some digital currency selling sites. But, remember that selling bitcoin is not bad, as long as it is continuously rolling in the market, globally.