According to the Epic Games Fortnite team said that
The PC version of Fortnite currently requires a graphics card capable of running Microsoft Directx 11. In v11.20, however, you will have the added option to play the PC version using the new support Microsoft Directx 12!
By using DX12, PC players will high-end GPUs may experience a higher and steadier frame rate. This is because DX12 delivers better CPU performance and allows for the distribution of rendering jobs across multiple cores.
In the future, Epic Games Fortnite team plan on adding features to Fortnite that are made possible by DX12. Allowing players to opt into DX12 before these Features are added will give them available information in their development efforts and help them shake out any bugs that have.