I find this interesting and it looks like an advise never to leave running market over to the next trading day, this is dangerous. Every trading day has it own challenges so never expect total continuation of the previous day. So close your running trade if on profit or you put stop loss if you are losing before the night falls.
We have scalping and day trading. If you are a scalper or a day trader, you have different strategies if you are compared to swing traders. There are some trades you can leave for more than a day. You may have the intention to leave it for some hours but later you lose and decided to leave it until it came to your profit on the next day (I know most traders do high risk trading which can liquidate their trading asset if leaving trades like this longer). But the way some traders open position with high leverage and no averaging may lead to liquidation or high loss of money. I always wonder why most traders talked more about stop lose when not needed for all types of trading strategies.
Surely stop loss is good for every trade order and strategy whether scalping, day trading or swing. A spill over running trade is preferred for swing trading but not scalping or day trading. The reason that it is preferred for swing is because of the long time that it is expected to last and within that time, a losing trade can revert to profit before it hit on the stop loss. But using low leverage is also good for every trade except for high risk traders and they should be ready for any liquidation any time trade go wrong.