Why did Russia want a notorious arms dealer released from US prison?

US basketball player Brittney Griner has been released from a penal colony in Russia. The two-time Olympic gold medalist, who had been jailed on drug charges, was released in a prisoner swap in exchange for Viktor Bout, an infamous Russian arms dealer.
Journalist Douglas Farah, author of the book "Merchant of Death: Money, Guns, Planes, and the Man Who Makes War Possible" about the arms dealer Bout, spoke to DW.