Sorry, meant to supply an answer but forgot.
Most modern mammals are unusual among vertebrates in that their teeth not only differ according to special functions, but that they fit into each other in ways that are adapted to their eating habits. This permits highly efficient adaptation to a wide range of eating habits. Interestingly, it must be an ancient bauplan because almost all modern mammals share the general scheme of incisors, canines, premolars, and molars in some variation (examples of such variations include: many herbivores leave a gap where carnivores have canines, and carnivores' premolars and molars usually are modified into carnassials). Exceptions to this rule, or possibly modifications too distant for us to trace the resemblance, include whales and various kinds of anteater-like species.
("Bauplan" is a term used in comparative anatomy and embryology. It literally means "building plan or layout" and refers to the way in which the anatomical structure of a body develops. For example all vertebrates develop with the central nervous system above and the main circulatory vessels below — that is part of their bauplan, whereas in the general arthropod bauplan those two are the other way round.)
Now, impressive though our dentition and that of most of our fellow-mammals may be, it entails certain problems. We can rebuild and reshape our skeletons as we grow, but our options for doing that with our teeth are very limited. Crocodiles and sharks can shed damaged teeth and replace them almost casually, growing larger ones as the animals grow. However, that does not work so easily for our scheme in which each tooth needs room to grow into a precise shape where it must both be the right shape for its function, and also fit in with its neighbours. Instead we mainly grow our teeth internally until our jaws are large enough to accommodate our adult teeth, all except our second and third sets of molars, which must await further jaw development. Those delayed molars are our 12-year-old and wisdom teeth.
That is our lot. We have to make them last because we won't get another set (not without some thoughtful application of technology anyway!)
Except for our last few centuries that did not much matter, because those teeth would work very well for as long as the rest of our bodies would last, so it did not much matter that our system of tooth production did not lend itself to constant replacement. I just cannot wait for the day that our genetic engineering technology will correct this shameful state of affairs.
So you can see that actually we really have only one serious set of teeth that we grow during our adolescence. As babies we don't need teeth for suckling, and in fact at that stage teeth would be a serious nuisance. However the limited period of suckling that we support leaves us with a gap between the time when teeth would be unwelcome and where they become vital. We bridge this gap with a set of rudimentary teeth, suitably sized and so shaped as to be cheap to shed and harmless to the generation of our permanent teeth below. Anatomically they do not match our permanent teeth in detail, but only in outline. Biologically they are teeth all right, but structurally they are more like sophisticated tooth caps.
This general scheme of things applies very widely, although with many variations. That is hardly surprising after all, when you consider how widely mammals vary in their adaptations to their lifestyles. Consider whales, elephants, rodents, carnivores and ruminants for just a few examples. Whales do not have milk teeth because they either have baleen or the widely spaced teeth not adapted to chewing. Most of the others however, especially those that are fairly long lived, do have milk teeth in some of the areas of their mouths anyway.
