The reason someone would suggest doing a late addition is because bittering hops will work better if the wort is at a lower gravity, and has less malt extract in it. This can actually make a pretty huge difference in some cases. I use the brewersfriend .com calculator and you can put recipes in there with "late addition" and you can see that sometimes the IBUs will increase by a significant amount. Whenever I'm brewing an IPA, I make sure to do late additions for the simple fact that I can get more IBUs with less hops so I basically save some money.
As ACbrewer said, another reason to do a late addition is for if you're only doing a partial boil (boiling 3 gallons and topping it off to 5 gallons in the fermentor), boiling a concentrated mix of extract can darken your beer. Best case scenario is to do a full boil, but not all of us want to do that so you can just add the malt extract late so as not to darken or carmelize the extract too much.
Some people may say not to do late additions depending on what extract you're using. Something about a "hot break" and whether the extract was boiled enough by the manufacturer.