Proper change control determines what is in and out of scope for the project early, and then takes steps to avoid scope creep or potentially harmful shortcuts. With proper change control, each potential change is weighed, and then if accepted, it is tracked separately from the project.
For example, I have seen software projects that have no scope control. The project sponsor sets a deadline and customers are expecting the project. Then, the project sponsors and clients keep making changes to what the program will do. On the day of delivery, the customers are not satisfied because the program is still in development.
Project change control assures that budgets, schedules, and deliverables will be met without changing the project scope. Proper change control leads to happier and more productive teams, since more projects will be deemed successful, leader to greater satisfaction and rewards.