Marsupials are primitive mammals that bear their young prematurely then shelter them in the mother's pouch (the marsupium) until they are fully developed.
In order to understand the evolution of mammals geneticists have to compare the genetic code of various mammals with that of fish and birds. But the genetic codes of most mammals are too similar among each other while being too different from those of fishes and birds. However, marsupial mammals and monotremes (egg-laying mammals) are primitive mammals that allow such comparisons.
The reason they are considered primitive in relation to the modern placentals is because, regardless of whether or not placentals share any ancestors at all with monotremes and marsupials, monotremes and marsupials retain many characteristics that were originally found in birds, reptiles, and the earliest mammals, including their means of reproduction, gestation and lactation, their dentition (in the case of the monotremes), and the articulation of their shoulder and hip joints (and some others); characteristics that were replaced with other (further evolved) characteristics in the modern placental mammals.