Here are the reasons I do it:
A - better hop utilization. This is debatable, and might very well be wrong, but I think my beers taste better with late addition.
B - no boil overs. With 3 pounds of malt in a 3 gallon boil, I never have a boil over. With 6+ pounds of malt in a 3 gallon boil, I had a hard time preventing boil overs. And I often lost. Which my wife hates.
C - drop the temp of the wort at flame out. When I add the rest of the malt, the wort stops boiling instantly. I've not measure the temp differences, but it seems like it takes me less time to chill my wort when using late addition.
I started doing it for A, but keep doing it for B - the less caked on crap I have to scrape off the stove, the happier everyone in my house is. FWIW.