From Chabad.org: On the day when the Tabernacle was inaugurated, the tribal leaders wished to bring inauguration gifts. Collectively they brought six covered wagons and twelve oxen to assist in transporting the Tabernacle when the Jews traveled. In addition, as representative of their respective tribes, they wished to offer individual gifts and offering. G‑d instructed Moses to accept these gifts, and that on each the following twelve days one of the leaders should bring his individual gifts. Although each leader brought identical gifts, the Torah describes each one individually.