This thickness reduces the danger of removing tannins from the grains as it acts as a buffer. From everything I've read and others have said on here, vorlaufing is not so much to clarify as to prevent tannin extraction.
The very low pH of the first runnings prevent problems with tannin extraction.
You're right, clarification is not the purpose. The goal is to NOT introduce any of the husk into the boil. I really can't say how useful vorlauf is, but I do believe that there is some advantage to keeping solid grain material out of the boil.