At the risk of beating the subject to death, let me expand upon what Brad says below.
Optics - When you see a color, such as red, you look through the paint and see a relection of the top color off the undercoat. If you want a darker red, a dark under coat will give you a red that is not so bright. If you want bright colors, a light primer reflects better and your colors will be brighter. Some artists feel that bright colors are not "realistic," so they go for the darker undercoats to make their colors more muted. This is particularly desirable for modern figure subjects, such as WWII, where the uniforms are drab and the colors muted. The same results can be achieved by simply mixing mutued colors, but dark undercoating is another way to do it. I actually have a flat on my bench right now that is undercoated in balck because all the colors of the figure are dark and muted - its the "Caretaker" from Ghost Rider. However, the flames on the head and extremities are underpainted in white to give brilliant flames and in contrast to the dark brown of the figure itself.
Paints - Dark primers work better with acrylics because the range of value extension in modeling acrylics colors is some 12 shades up (lighter) but only 3 shades down. In other words, the are generally intended to work from dark to light. This is not true with oils, where it is much harder to bring a dark color up by painting over it. I'm not sure about enamels, but assume they tend more towards acrylics, based on my experience. For me, as an oils painter, a white or light gray works much better for historical subjects and for fantasy subjects becasue I can achieve more brilliant colors - which IMO gives the figures a nice color "pop".
Style. War gamers use black undercoating as part of painting style. They paint the highlights (at least this is one style!) and let the blacks be the shading. This gives a very contrasty look which works well on smaller figures but (IMO) looks out of place on larger pieces. However, if you have to paint a few hundred figures in about a month, this is a fast way to amass a relatively good looking painted army quickly.
How you see. Some painters paint from dark to light (shadows to highlights), some from light to dark, and some start in the middle and work to both extremes. If you are not a dark to light painter, you will find it harder to anticipate your highs on a black primed piece. If you start dark, then black primer will be in line with how you see and paint. The converse is true for a light primer if you are a dark to light painter.
Color mixing and transparency. It is easier - and requires less paint -to mix a color from light to dark. For example, if I want a dark gray, I start with white and add black. I can get the same thing starting with black and adding white, but it requires more paint. The same is true with painting on top of black - it requires many more layers of any color paint to brighten a dark spot than to darken a light spot. So you're making a lot of work for yourself to prime in black if you have to lighten the value of whole figure over the black. If you have a dark subject, then your work is reduced becasue the darker values are already in place. IF you paint in washes or use transparent colros, these techniques depend on the color underneath to be effective - so for light subects use light undercoat; and for dark subjects use a balck or dark gray.
Bottomline - There's no right way or wrong way. You can achieve any result a number of different ways. It's what works for you - and to a certain degree your style and subjects. Since most of us are not trained artists, we tend to go with a technique without necessarily understanding why it works or does not. The longer we paint, the more we learn, and the better we get at picking techniques that achieve the results we are after.