Those who saw the movie in theaters got a voucher for a free copy of the World of Warcaft Battle Chest (The full game up to Mists of Pandaria) + 30 days free game time. The home release version comes with that plus a Medivh Hearthstone Card and Gul’dan as a playable character in the game Heroes of the Storm.
If you want to try WoW for 30 days no strings attached, there’s no harm in redeeming the code. You can decide later if you want to subscribe and get the next two expansions (Warlords of Draenor and Legion) later. Even if you decide not to subscribe you can still play WoW for free: Accounts without an active subscription become Starter Editoon Accounts. You can’t level past 20, play a character over level 20 or use almost all of the communication features in the game including the Mailbox and Auction House. Aside from that you can still explore Azeroth unrestricted.
The Warcraft movie will be released on Blu-Ray, DVD and via online stores (iTunes, Playstation Network, Xbox Live, etc.) on September 27. I didn’t see the movie in theaters so I am looking forward to watching it ^_^