Why do professional bodybuilders aim for 15 reps?!

Why do professional bodybuilders aim for 15 reps?

I read in an interview about Jay Cutler that he aims for 15 reps. I've also seen him and other bodybuilders workout and they aim for 15 reps usually.
Do these guys know anything about bodybuilding or do they just inject steroids in themselves?
Why do 15 reps when trying to get bigger?

I see Jay at the gym all the time and he does not only train only in that rep range. bodybuilders are always changing the number of reps that they perform depending on what phase of training they are in. a time under tension of 45-60 seconds per set is optimum to stimulate hypertrophy. this comes out to about 6 seconds per rep or the 8-10 rep range per set. so for a bodybuilder the 8-10 rep range is actually low so the 15-20 rep range is more in the medium category and high reps for bodybuilders is from about 20-50 reps per set. a higher rep range is needed to efficiently fatigue the slow twitch, intermediate and fast twitch muscle fiber types.

regardless of the rep ranges that are used during training the diet ultimately determines the size of the muscles. and this is exactly why 95% of the people that train for size never achieve their goals. going to the gym and training is the easy part but consuming 5-7 healthy meals daily for years on end is not.