The theory is that our deltoids have more Slow Twitch (type l) muscle fibers than the average muscle groups and they supposedly respond better to higher reps, than lower.
When I train my delts I use primary 16-20 rep range and for me it has provided the most success, in addition to decreasing my occasional shoulder pain.
If you think about it, you already indirectly train your front & rear delts with heavy low rep range, every time you train your back and chest, which targets your fast twitch (type II) muscle fibers.