The universe is moving towards entropy. If you don’t apply energy to something, it will become increasingly disorganized and disorderly. Entropy applies to your project as well.
Most projects will feel completely orderly when you first start them. You will meet with the project sponsor (the person who tells you to do the project.) When you do, it will seem that everything fits just right. Even if you have a large project team, everything will feel orderly for a while. But slowly, entropy will take over. Someone will start another project, and some people will turn their attention to that work. A crisis will loom, and other people will go in that direction. Someone will have a week of sick time and not be able to keep up. Another key contributor will find a job somewhere else. Soon your project will be a picture of disorder.
Spending energy to manage your projects brings order back to your project. The more energy you spend, the more orderly your project will be. When your project is orderly, It is most likely to succeed.
The one reason to manage your projects is to create an orderly project with a good chance of success. Everything in order means energy that will change the world.