This is called "Late Extract Additions" and some people prefer the results over boiling the entire amount of extracts for the entire time. It is supposed to help keep the beer from darkening too much. Extracts have a tendency to be darker than All Grain.
It will also change the flavor a small bit. And it will allow the hops to isomerize a bit more, so you may have to adjust the hops.
I would add it in during the last 15 minutes. This won't make a real impact on teh flavor of the beer, yet still provide for more sanitizing, and give you a few more minutes to prepare the chill if necessary.
If your takin' your girlfriend out tonight
You better park the car well out of sight
'Cos if they catch you in the back seat trying to pick her locks
They're gonna send you back to mother in a cardboard box !