For example BTC is trading at 8000$ and I put up a sell order at 9000$, most times this won't happen in an instant, sometimes you wake up the next day and your order became successful isn't it? Now for those saying 'don't leave your coins on exchanges' how do you trade?
Somewhat we have similar strategy in trading. Putting sell and buy orders wat above and below the peaks. I gain a lot of XRP butting way way below its buying price and twice I have bought with it. That was a very big discount and my holding has tremendously increased.
I also have some of my crypto in exchanges. This saves me some transaction fees than transferring cryptos to wallet to exchanges.