to boil or not to boil?

[discokid2k]

Hi All,
I have a questions for some of the home brew pros out there. I have done a few recipes that call for the extract to be added at the end of the boil, but most you add the extract at the begining. Why are the reasons one would add at the end of the boil instead of the begining? What qualities does this add or take away from? I am just curious how this affects the overall flavor or complexity of your beer.
FYI, the only brew I have done that has one add the extract at the end of the boil was a belgian specialty(somewhat similar to a "Blue Moon" recipe, only more hoppy and stronger.....)
Can anyone answer this?
Thaanx from an ametuer going pro>

 

[McGarnigle]

If you're using a partial boil - e.g., brewing 3 gallons with the intent of topping off with water in the pail - you are changing the gravity of the wort. This affects hop utilization: the heavier the wort, the less you get out of hops. So people hold back some extract at the end to counteract the fact that they'll be topping off with water.

Late extract additions also keep the beer lighter in color.

There are also aspects related to caramelization, which may be a good thing or a bad thing depending on the style. If you brew a heavy wort for a long time, you get some caramel notes added to it.

 

[cappy]

Add it later if you want a lighter color beer.
It will affect your hops though. If you are using a kit, they would have accounted for it.