why is it important to define the scope and perform requirement analysis for the project? How can scope changes be effectively managed after a project has started?

why is it important to define the scope and perform requirement analysis for the project?
How can scope changes be effectively managed after a project has started?

How can scope changes be effectively managed after a project has started?
Provide at least 1 example of a project that you know of where the scope has changed after it started.

How was the scope change managed?
What were the results?
In your opinion, how did the scope change impact the project?