Because i need to see by time to time if my bitcoins are gain some cash or down. Ill spend time because i need more profit and i want more bitcoin in my wallet and sell again if bitcoins are in a big profit . This time i collect more bitcoin and wait for the big profit.
The market price for all of the cryptocurrency is depending on supply and demand so you cannot always expect to earn profit, long term trading will be your situation if you want to invest on cryptocurrency.