Good question, Mochi. If you get a tattoo on the nape of your neck, you know that other people can see it, so you are probably deliberately going for a certain "look". I think of it as analogous to people who are very concerned with their "rear view" in jeans. You can't see your own butt without a mirror, but everybody else can.
If the tattoo is further down the back, you may or may not have got it for the "look" it gives, since most back tattoos are covered in normal clothes. Sometimes people like the idea of having a tattoo that they can easily conceal, but that they can show if they want to by wearing a skimpy top/bathing suit/no shirt at all (mainly guys for option 3, lol). Others might get a tattoo more for its symbolism -- that is, for more personal reasons -- and they just feel like the back is a good spot for a tattoo.