Projects don't exist in a vacuum. On the contrary, projects are part of an organizational continuum. Therefor the purpose of a project is not the project itself nor is the the sole creation/production of the project deliverables.

What ever gets created/delivered by the project has a purpose in that continuum. When starting up and initiating a project don't focus on the output alone, ask yourself:

"why do we/they need that project (output)?"

Below is a diagram that shows the relation between the project output, what you as a PM are expected to deliver and the project benefits, what your project sponsor wants to achieve with it.

If you make it a habit to draw this simple diagram at the start of each project and discuss it with your sponsor, project board/steering committee and your team then it will be much easier to avoid scope creep and keep your team focused.

It will also be valuable after the project in assessing the success of the project.