Scope Control and Change Control
If you use the WBS approach to building your schedule, it is much easier to control the scope of the project. The 100% rule defeats the "as long as you are there" phenomenon: the tendency to add features that are not within scope because they would be nice to have.
Using the 100% rule allows you to perform top-down planning and budgeting and also enables tracking and reporting at the work package level. This is essential if you will be using any type of the earned value reporting in your project. You will be able to baseline the project and keep track of history at a work package level rather than trying to manage at the individual activity level.
Change control should also be viewed at the work package level. Budget and schedule impacts are easier to control in logical units, and it is also much easier to see the ripple effect of a requested change at the work package level.