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.
Bitcoin is money thats why people sell also they need, as we know bitcoin is just a matter of patience so that money will more bigger in time but if time that money hardly needed so we need to sell and also people sell because maybe they find other one that needs more money that they wanted to hold.