They provided the power needed by the CPCs, so that you didn't have to carry the monitor around just to have power, and a TV-modulator which modulated the monitor signal in a way that it could be seen on a regular TV-set.
Having an MP-1/MP-2 today is more or less just a gimmick, as a self-made [[TV SCART connection cable]] gives better picture quality (SCART connectors weren't common back in the 80s) and the power can be taken from a PC AT or even ATX power supply as well.
The difference between the MP-1 and MP-2 models is just the additional 12V power supply for the CPC664/CPC6128's disk drive in the later model.