It lightens the beer, primarily, and may also reduce some caramelly flavors. Add the extract with ten minutes to go (enough time for it to sanitize), although some (notably Palmer, IIRC) say that you can add it even at flameout and the residual heat will be enough to sanitize.
Just as important, since your hops will be boiled in a lower gravity wort for most of the boil, utilization will be a lot higher - a considation if making something like an IPA (where you want those hops) or something like Cheese' Caramel Cream Ale, where you do not.
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