It really depends on the size of your boil. If you're doing something close to a full boil (5-7 gallons) then it really won't matter much about adding some extract late. On the other hand if you're doing a partial boil where you might be only boiling 2 gallons of water, it can make sense to do a late extract addition.
This would help you get better utilization of your hops (because the gravity of a partial boil would be higher) and also keep you from possibly scorching some of the sugars while trying to add all the extract to the initial boil.
So to answer your question, the main reason to add some of the extract later in the boil is simply to keep from burning some of the sugars you might get from adding everything to a partial boil and to help keep your hop utilization as close to accurate as possible.
You'll still make excellent beer either way, but just some things to consider.