Masterkey suites make a convenient way to access numerous doors without having to carry all the individual differ keys.
It can be used to control who can have access to certain areas. It is also possible to have various key levels (sub masters) within a master key suite.
For example you could introduce sub master keys which would control certain areas, i.e. four different buildings could use a sub master key for each building, with a grand master key that opens all locks in all four buildings.
See the example right for a typical design layout for this style of suite.