I have used this tactic over the years on soraka, taric. It is really important especially if you are controlling the lane since you put your opponent back yet even further behind. Typically you would only do this if you are fine sustaining your mana, have no reason to believe the CD will not be back in time, and if the enemy bot lane has weak pressure due to them being bad or you guys just have really good pressure. Benefit = your adc gets his item first if the lane is well and the denying is done correctly. Or prevents the enemy from getting a couple hp pots, which does make a significant difference in terms of controlling the lane or having their supports do more such as expeling more mana. So, yes the benefit is actually quite high.