Numbers 17:4

And thou shalt lay them up in the tabernacle of the congregation before the testimony, where I will meet with you.

The command was to lay up the leaders' tribal rods in the tabernacle of the congregation before the testimony, where God would meet with Moses. That placement put the rods before God in the place of meeting, so that He Himself could make clear which man He had chosen for the priesthood. It was a setting for divine choice, with the question of the priesthood laid directly before God to decide.

There, overnight, Aaron's rod blossomed, showing that God had appointed Aaron and had chosen Levi's family, and thereby establishing the priesthood. That blossoming settled the dispute and question over who was appointed to the priesthood. The rod was then kept in the tabernacle as a lasting witness to future generations, a standing sign against rebellion and murmuring.