I will give you an example, lets say you are in a trade which you think will make say 5 points. You scale out 1/2 your position at 3 points and bring your stop to break even.
There are now 2 possibilities after this occurs.
1) The trade goes all the way to 5 points and you made money.
2) The trade goes back to break even and you made money.
Now lets assume you did not plan to scale out, the the same trade goes to 3 points.
There are now 3 possibilities after this occurs.
1) The trade continues on to the 5 point target you made money.
2) The trade reverses and you get stopped out at a loss.
3) The trade reverses, you are so upset that you are about to get stopped out on a trade that could have been profitable, that you remove your stop, and start adding contracts since you know it will go back up and you can make your money back. However, you were glued to your chart and did not see on the news that North Korea just launched a nuke at the united states causing you to blow out your account even though we were able using laser guided rockets to explode the nuke harmlessly over the ocean.
So in conclusion while you might have made more money by not scaling out, you can also have a better money management system in place by using a scale out strategy.