Genesis 38:8 - Ellicott's Commentary On The Whole Bible

Go in unto thy brother’s wife. — We learn from this that the law of the Levirate, by which the brother of the dead husband was required to marry the widow, was of far more ancient date than the law of Moses. Its object, first of all, was to prevent the extinction of any line of descent, a matter of great importance in those genealogical days; and, secondly, it was an obstacle to the accumulation of landed property in few hands, as the son first born after the Levirate marriage inherited the property of his deceased uncle, while the second son was the representative of the real father. A similar custom existed in parts of India, Persia, &c, and prevails now among the Mongols. The Mosaic Law did not institute, but regulated the custom, confining such marriages to cases where the deceased brother had died without children, and permitting the brother to refuse to marry the widow, under a penalty, nevertheless, of disgrace. Onan, by refusing to take Tamar, may have been actuated by the selfish motive of obtaining for himself the rights of primogeniture, which would otherwise have gone to his eldest son, as the heir of his uncle ‘Er.