A master key system means that it is possible for people, who are authorised to access different doors within a system, to each have just one key.
Benefits for you:
Security with as few keys as possible
Easy use, as only one key is needed for all relevant doors
Simplified organisation
It is possible to define exactly who has access to which areas
All cylinder locks and types (main entrance doors, internal doors, garage doors...) can be locked with just one key