What is the purpose of the Vorlauf?
 

Why vorlauf? What is the advantage? Won't any debris from the mash just settle out in the kettle or even more so in the fermenter? I'm batch sparging if it makes a difference.

from the wiki:

http://www.homebrewtalk.com/wiki/index.php/Vorlauf

You want to minimize grain husks in your boil because they give off sharp, bitter flavors at high temperatures. Best to reduce that as much as possible. Vorlauf also helps prevent stuck sparges. Batch sparging just means you'll have to vorlauf twice.

I think if grain gets into the wort and is boiled for the 60-90 min you wouls extract tanins and get off flavors in your end product. The vorlauf greats a filter out of the grain bed and should be done with each sparge as well.
This is an assumption on my part...

In a decoction, husks are thoroughly boiled. So I doubt that's a major concern here. I think clarity is probably the biggest reason, but you're right - just let that stuff settle out as the beer cools and ages.

Personally, I recirculate about a gallon mostly because it helps the grain bed settle a bit... then I can fit more sparge water on top.

yep. I agree with you. Many people "claim" tannin extraction and off flavors from not vorlaufing, but there's been plenty of discussion on here about people that forgot to vorlauf and still wound up with a great beer.

It's actually a little different. Given that the husks have the wort sugars to buffer them in the decoction, they don't leech tanins as they would without the sugars. That's one reason why fly spargers monitor the gravity; once the amount of sugars drop, you risk leeching tanins around 170*. But with the sugars, you can certainly boil the grain without any astringency.

How do these BIAB guys get away with what they do. It's not like they can vorlauf. So is it really essential?

Fine mesh bag captures those small particles of grain.

So why not put a mesh bag over your manifold and skip the vorlauf? I'm just asking.