Why? So that Depardieu can pay tax at 13% in Russia (subject, of course, to not living somewhere else that makes higher claim upon him).
According to the Guardian Putin intended this as "a bloody nose" to France. If so he's failed. All he's done is confirm he's willing to debase the sovereignty of his own country by doing grubby deals in the tax haven market where nothing is of value bar the right of the individual to abuse the state of which they are a natural part.
One would hope Depardieu would have the sense to decline the offer, but given it's Depardieu we're talking about that's a very limited hope.
He and Putin seem by their actions to share in the depravity of tax abuse whilst celebrating the idea of citizenship for sale to ensure the abuse of those states that accept their obligations to their citizens, a concept clearly beyond the comprehension of either of them.