I think, firstly the right knowledge and understanding of a particular trade should be noted and continuous practice (demo) and to know how a particular trading works and it's pattern , all this comes with continuous practice (demo) even before using your money to start a trade...understanding and right knowledge really matters....
Even after the pose of being professional trader, sometimes we will still need to acquire more advanced knowledges as long as the market remains unstable with different dimensions of trend lines.
And of course when test running what we have not put to practice especially after locating a new trend, using the demo for the practice is good as it would safe you regrets if Incase of lost on the process before then trading with your real money.
That particularly will help you cut lost even at your continues practices while you will be more conscious at when you will trade with your real money.