THE FIRSTBORN HAS KINGSHIP RIGHTS

Midrash Rabbah - Numbers 6:2

TAKE THE SUM OF, etc (Numbers 4:22).

… But with kings upon the throne (Job 36:7) signifies that the Holy One, blessed be He, allotted honour to the firstborn and by them kingship should fittingly be assumed as it says, But the kingdom gave he to Jehoram, because he was the firstborn (II Chronicles 21:3). In the case of David it likewise says, I also will appoint him firstborn, the highest of the kings of the earth (Psalm 89:27). This explains, ‘But with kings upon the throne.’ He sets them for ever (Job 36:7).