Because this is a business. and business should bring profit .. if the price of a coin is higher than the price of ICO, why not sell it? And I sell my coins to gain a profit which will allow me to gain more Bitcoin.I have also been selling coins to accumulate Ethereum.If Either Bitcoin or Ethereum start going too high I will step out and try to get more at a cheaper price in the future.
I sell my coins to earn good profits but it depends on the market fluctuation, there's a perfect time for selling coins. If your targets is much higher then hodl for a long time for that and always monitored the markets volume.