Question about veins

Why some people have very pronounced veins (usually on their hands) while others do not? I am sure body fat percentage plays a role, but still there are some really thin people with no veins protruding.

Do pronounced veins have any advantage/disadvantage? Are they indication of some condition?

IANAD, but I suspect that your original suggestion that this difference is caused by variations in the skin’s fatty layer of protective tissue, is fairly close to the mark. I have pronounced veins on my hands and relatively little apparent sub-cutaneous fat. This feature is also more common in older people as the skin thins during ageing. I suppose hypothetically it puts me at a marginally increased risk of blood loss through injury and risk hypothermia, though on the other had it might keep me from overheating in hotter climates. It also helps when newly trained nurses are attempting to find a vein. I’d hazard a guess that certain skin-affecting medical disorders such as Ehlers-Danlos syndrome or Marfan’s syndrome could potentially cause such an appearance.