The 203fe's meter display is in the body, at the top of the screen.
The PME51 displays EV at the bottom of the screen.
The PME51 has the cutout for the 203fe, so you can see both meter displays. You basically have a choice.
The 203fe meter does not work in "C" mode (or early versions did not) There is no EV read out for non-E lenses. The PME51 works better in C mode with those lenses
If you had a 500 series body as well, the PME51 would make more sense than getting a PM51 for the 203fe and a PME51 for the 500. I suspect most pros shooting with a 200/2000 body probably had a 500 body too.
The PME51 was introduced with the 205TCC, which only had a spot meter. This gave that body a center weighted average meter option.