Because it is for candy (which btw I do not make but listening back to my mom) I take it you are not holding it at that boil? Just bring to the boil and take off the heat?
I'm guessing reason is NOT to walk away!
It will come to the boil faster with the lid on but also as soon as it starts to boil, as I'm sure you already know!, it immediately starts to climb up the pot - to boil over.
Perhaps they are trying to avoid that and the rising temp which could likely make your candy too hard/crystalize/and or just plain not gummy!
Meddle not in the affairs of dragons for you are crunchy and taste good with ketchup.