End result difference in the bread with variable hydration starters.. ??

So I've been following this Reinhart recipe for a while now tying to nail down some basics, and it got me thinking about some things this morning. I did a search in general on the web and again on this site, but couldn't find quite the thing I was asking.. 

In this recipe, he's got us whipping up an ammount of 125% sourdough starter, keeping it at room temp for 4-6 hours and then retarding it overnight. The next day, we turn that into a "firm starter" by incorporating pretty much only flour.. leaving at room temp for 4-6 hours or so and the retarding overnight. The next day (the third, now..) we build the final dough with this grown up starter and proof it for a while, shape and final proof, and then retard again overnight.. baking the thing the next day, four days total. If you already have a 125% starter, I supose you could knock a day off this schedule, which I'm considering doing.

Anyway, my question is, what's the basic idea behind the different starters? I can't seem to find a good reference for what happens *differently* in a more liquid starter than what happens in a firmer starter. Basically, does a liquid starter favor certain characteristics, while a firmer starter favors other characteristics? If so, which is which, so I can wrap my head around that to use to my advantage. If a book reference could get me to the answers I need, I'm happy to do the reading/legwork.. thanks!