I'm not an expert on this, but briefly, you're right.
There seems to be a great deal more to it and practitioners make all sorts of claims for its versatility and subtlety.
As well as control over overall grade and the distribution of tonality, dodging and burning can be done with either filter for very fine control. I have even heard of flashing...
If you've read of Steve Sherman on this forum, he uses blue and green filters for the same purpose, as did the out-of-production Phillips enlarger. They are the complementary colours to magenta and yellow. Mr Sherman has originated and advocates an entire system of exposure, development and printing. He also wears a baseball cap in his darkroom.
The more common magenta and yellow filters are adjusted to give even grade-spacing and constant exposures when changing grades.
Although split-grade printing has been possible since variable contrast paper was introduced in the 1940s, it's only since exhibition-grade papers have become available that it seems to have become so popular.
There is a good deal information available on the web, if you want to know more.