To date, Apple has required that all games released for Apple TV must support the basic Siri Remote. Apple has now reversed stance, allowing developers to require a real game controller. It paves the way for more advanced games on Apple TV.
Games can require a game controller
The Siri Remote has a touchpad and a few active buttons that you can use in games, which has limited how complex game controls can be. It has also disqualified certain types of games.
To be clear Apple TV 4 has supported external game controllers since launch and developers have been able to offer optional support. The change is that game developers can now develop games that ditch support for Siri Remote and require the use of a true game controller. You can buy one of these MFi Bluetooth-based controllers separately.

- "When designing a tvOS game, you may require the use of an MFi game controller, but where possible you should also support the Siri Remote," Apple’s developer documentation reads.
The new policy should pave the way for more ambitious games with more advanced controls on Apple TV, including games in genres such as FPS, sports, and RPG that previously were not possible to port to Apple TV because of the Siri Remote policy. The A8 chip in Apple TV is capable of running some of the previous-generation console games.
- Source: AppleInsider