It's a cash out opportunity for some but for others it's a medium of exchange. There are also quite a lot of ideological people.

If you believe that Bitcoin has a bright future and even may become a global currency, it's a bad investment move to sell your bitcoins at any time. Sure, you can argue that you can sell high and later buy more for less but that's timing the market and no one can actually reliably do that and as a result if someone believes that BTC will be huge in the future will never sell. When the mission is accomplished, they will use it as money to purchase goods and services.