Hi Satya,
It's part of the Monitoring and Control process. When your team is executing the plan and working on the deliverables, you need to keep a constant lookout for any potential problems. That’s what the Monitor and Control Project Work process is for. When you find a problem, you can’t just make a change...because what if it’s too expensive, or will take too long? You need to look at how it affects the project constraints—time, cost, scope, resources, risks, and quality—and figure out if it’s worth making the change. That’s what you do in the Perform Integrated Change Control process. After Perform Integrated Change Control, we go back to the Direct and Manage Project Work process to put the approved changes in place.