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.
They sell there bitcoin to use in their own expenses if they needed and they trade their bitcoin to invest coin to get huge profit.
yes people sometimes sell their bitcoin to be invested into altcoin so that the maximum profit they get and in my opinion it's even a clever action than just for fun without any benefit even though it is also needed