Why do some surgeons use drains after Breast Reduction Surgery and others don't?

The use of a drain is largely related to the amount of breast tissue removed. The more that is removed the more likely a drain will be necessary. It does not decrease the incidence of postoperative bleeding (hematoma) however, it does decrease the the edema (swelling ) in the area. 

I find drains to be very useful after breast reduction.  There is very little downside to using drains.  However, fluid collection after surgery which can result from not using drains can be a significant problem.

Many surgeons use drains because that's just what they do.  That's how they were taught and that's how it's going to be.  Most surgeons that have trained within the last 10 or 15 years use drains when they are needed.  Decisions that tend to lead me to use a drain would be if there was abnormal bleeding/oozing during the case or or if multiple cysts were ruptured during the case, and I was concerned about an infection.  Other times I may use drains would be with the shorter scar breast reductions that limit scarring but have potential to leave a 'dead space' (an open area) within the breast.  Your body will not tolerate such a space and quickly will fill the area with fluid.  In that case a drain would be used.  All that being said, drains are not the end of the world, and after a couple days they will come out.  If you are comfortable with your plastic surgeon, and you trust him or her, then go for it.  Best wishes.