3D modelling software has had a long history of expensive, esoteric, difficult-to-use applications. Epic, hoping to make money from large numbers of Unreal Engine licenses, would like to see that situation change. Ideally (for them), 3D modelling software would be free and easy to use for everyone, thereby lowering the barrier to entry for those who might make 3D games using UE.
By donating to Blender, Epic hopes to achieve two things:
1. Blender becomes better and easier to use, allowing more developers to enter the market and buy UE licenses.
2. Increased competition puts downward price pressure on commercial 3D packages such as Maya, thus making it more affordable for small-to-medium sized game studios, potentially freeing up budget for more UE licenses.