... I don't understand what the part markers are used for other than directing the eye.
Part Markers change the substyle being played at the bar in which the Part Marker is invoked.
There can be two substyles or more in a Style file.
Let's keep it simple for now, and I will describe what happens in the typical two-substyle Style file.
A Blue Part Marker happens when you click on a bar number once.
That would force the "A" substyle, which is usually going to be a simpler style, as might be desirable for the first whole chorus, or more succinctly the part of the song where the Melody, the singing takes place.
Clicking on the number again will turn that Blue marker to Green, which indicates that the "B" substyle will be used. The B substyle will usually be a bit more busy, especially as regards the Bass and Drum tracks. Quite often the Bass will change to a "walking" bass in the B.
Part markers also force a Drum Fill to happen in the bar before the Part Marker, as this is what good drummers will typically do when the band changes from one substyle to another.
Experiment, try listening to a bar with no part marker, then change it to Blue and hit Regenerate and listen again, then change it to Green and listen again, etc.
You can also place Part Markers of the same color one right after the other, too. In this case, the band won't change the way it plays, but those extra Part Markers will force that drum Fill to happen right before every part marker, making for a bit busier drum part as the song progresses.
--Mac