Project Planning Question:

Agile methodology is used to minimize the risk or surprise at the end of the project.
A project is divided into a small projects which has its own SDLC. In case of any failure, the impact will be very minimal i.e. impact will be only on that specific bit of the project.

Scrum is to maintain the transferancy of the progress with the stake holders and to cross check the achievements of the milestones.