So no gifts were necessary. Yet they were brought. Why? Perhaps because knowing we were loved drew us to return that love. We were blessed as a people; as a people, we would return that blessing with our own offerings, tribe by tribe. The gifts would benefit everyone; without them, there would have been no sacrificial rites. These last two actions in Naso were reciprocal: a gift of blessing for the people, a gift of offering for God.
It occurs to me this also represents the sealing of a contract. In my (extremely limited) understanding of contractual law, contracts can be (most easily) broken without consequence if one side has yet to provide any of the goods, services, etc. promised; just return what you got and we're on our seperate ways. But once you have given me your goods and I have given you mine it becomes much more complicated. Think of buying a new tv; you can change your mind at the checkout and walk away no problem, but once you've paid and brought it out to your car, the return process becomes much more complex.
G-d gave to us blessing; we gave to G-d physical gifts. Contract entered, signed, and sealed. We're bound to G-d now, and it'll take more than showing a receipt and paying a re-stocking fee to get out of it.