This video on how to use a vein viewer was interesting. The hand-held option seems less useful, as having it mounted frees up both hands for other work.
No phlebotomist has ever needed one with me, since my skin is as thin and translucent as an Asian dumpling. I suppose it would be helpful for those with poor circulation, low blood pressure, darker skin tones, neonates, or maybe intravenous drug users.
Image: YouTube / Greiner Bio-One International