A Master Plan is simply an overarching concept / strategy / guideline for a club or course that all future development projects should adhere to.
It ensures a consistent approach that should save money and mistakes.
General examples of clubs who are keen to do work but are without master plans are those where every new captain comes in and wants to get his pet project approved, quite often in a completely contrary style or method than those that have just come before him... That’s why it’s useful for a lot of clubs to have a consistent consultant architect also: as much to come once a year and stop the club doing something they shouldn’t.