I would like Hulkster to explain how important the MM pose is to a bodybuilding contest especially a 90s contest when it wasn't even judged back then.
since you are new, I will go slow for ya.
simple: the most muscular pose highlights the upper body (mainly chest and arms)
It showcases the detail in the arms and chest as well as the shape of the arms and chest.
If you have great arms and chest, you will have a great most muscular (see Ronnie, Arnold, Bertil Fox).
If you don't, your most muscular will be sub par:

It was long over due for being instated as a mandatory pose for these reasons.
In some cases, it can also showcase the quads, but most of the time pros tend to concentrate on flexing the upper body only.