Just to add to what Tom777 said, the more extract that is in your boil for the full duration, the more likely you are to carmelize some extract. This can possibly effect final gravity, and of course color and flavor as stated above. Out of habit and for repeatablility (sp?), I always do late additions, and it saves hops!
Kelpie Home Brewery, Lafayette Colorado
Primary: Creek Dog Stout x2, Two Dogs IPA, Grinning Kelpie Wheat
Secondary: empty :(
In Bottles: Belgian Wheat