What you get at the end of a project
A working app is the visible part. These are the things that determine whether anyone can maintain it after we leave.
How we estimate, and why the number has a range
A single date is a promise about things nobody knows yet. A range is a description of what we actually know.
What a good project brief actually contains
You do not need a spec. You need five things - and the one most briefs leave out is the one that determines the estimate.