One of the most important things to understand about the architecture of MPF is that the core MPF game engine is completely separate from the process that controls graphics and audio. We call the thing that handles graphics and audio a "media controller." The game engine and media controller talk to each other via something called "BCP" which is a protocol we created for this purpose which stands for "Backbox Control Protocol". (More details on BCP are available at the MPF developer site.)
Here's a diagram that shows what each piece does:
Why are the MPF game engine and media controller two separate processes? Two reasons:
First, having two processes means that each one can run on a separate core in a multi-core host computer. This makes efficient use of hardware since the trend is to have multiple cores. If the game engine and media controller were combined, then your quad-core Raspberry Pi 3 would have all the MPF stuff running on one core while the other three cores were wasted doing nothing.
Second, having two processes means you can replace MPF's default media controller with something else if you want different features. For example, there is a group of people building an open source Unity 3D-based media controller <unity_bcp_server> which can be used for very advanced 3D display graphics.
mpfmc unity_bcp_server mpf_and_mc_different_machines multiple_mcs creating_your_own