late addition of extract is to keep your beer lighter in color, and not get as much caramelizing of the sugars. it also helps with hop utilization, so you can get to the desired IBU with less hops. when you add all the extract, the wort is very thick and it doesn't pick up the acids from the hops as easy. the thick wort also undergoes Malliard reactions, darkening the color, and can burn some of the sugars, leaving the beer sweeter and caramelly. Adding half of the extract late can help alleviate this. usually in late extract addition, you add 1/3-1/2 of the extract at the beginning of the boil, then add the remaining amount with about 15 minutes left in the boil. this allows it to mix in and be pasteurized.
other ingredients that call for late additions are usually hops or spices. in the case of hops, if they are added to the boil for the full hour, the acids in them are converted to iso-acids, which give the beer its bitterness, but most if not all the flavor and aroma is boiled away. so you add some hops at the beginning for bitterness and some later, to add hoppy flavor and aroma. flavor hops are usually added with 20-30 minutes left to boil and aroma hops with 15 or less. spices are usually added with 15 minutes left to boil, enough time to pasteurize them and extract the flavors, but not so long as to have the flavor boil off.
