I agree with your general sentiment of the mediator pattern not being strictly necessary every single time. However, if you look at it from an architectural standpoint, the mediator pattern implements a sort of request/response or messagging pattern in the application layer. Having this pattern, including all its middleware functionality then allows teams or developers to develop very clearly defined functionality. This paradigm can be very useful and productive, though obviously, it can be implemented in other ways than through a mediator, too.