Genetic chimerism is a condition in which a single organism is composed of cells with more than one distinct genotype. In animals, this means an individual derived from two or more zygotes, which can include possessing blood cells of different blood types. Elves inhabit this world, but do not possess magic in this reality. Instead, they are all what can be genetically considered chimeras because they have a strange way of reproducing.
When partners decide to conceive, A male must add his genetic material at different intervals throughout several years to the female. Genetic material could come from a single male or various ones with no connection to each other. The developing child is slowly "built" with the genetic material of one father, or the makeup of several different fathers. After enough "material" has been collected, the embryo becomes fully formed and the gestation process can begin. Because of this, the nuclear family took a different route, with polyandry being more common.
The wife of a household (usually of royal blood or high class individual) would marry multiple husbands. This would usually be two, although wealthier wives might have more if they can support them. These husbands would usually be related (brothers, cousins, etc), but having no blood ties with the woman.
This genetic anomaly among humans is the standard norm among elves. What advantages would this provide to elves as opposed to other single-insemination species?