Change is an inevitable part of project management leading to changes in the project scope. Normally it is strongly recommended to define the project scope in advance before the launch of a project and not change it mid-way. But in every organization things happen: clients change their minds, stakeholders disengage and senior management change boats. And sometimes the economic situation changes.
So, if change happens, the project scope should be changed by a formal change control procedure in order to make sure the project will still be delivered on time and within budget.