Think about it. In a $5000 demo account, if you make $10,000, it's cool, but not real. If you lose the entire $5000, no problem, just open a new account.
Assuming a "perfect broker" with nearly infinitely deep pools of liquidity (so all executions match demo exactly - not ever going to happen in the real world), if you put $5000 of your hard earned dollars into an account and have a trade with a target of $100 make 50 dollars and start to retrace, the urge to close NOW to save the profits will be there, even if your trading system that worked on demo didn't call for this. A trade that starts off with a planned $50 SL reaching -$40 even though your gut tells you it's going to turn back in your favor often leads to SL's being extended (or even removed). These two urged in conjunction can lead to wrecking a profitable trading system by closing profitable trades too early and letting losing trades go too far.
On a demo, it wouldn't matter. When it's YOUR MONEY, the viewpoint changes.