One reason I was told was to give the aircraft more speed stability as at light weights and flap 30 it can be difficult to slow up and speed control is easier on final approach, especially in gusty conditions. I have to say it was not borne out in my experience- I never understood the logic of flying the aeroplane in an unusual configuration in the hardest circumstances only. In windy/sheary weather, if doing a low go around, I would not want that extra drag- I'd rather be climbing away as quickly as possible, and starting at flap 40 did not strike me as a good idea.