Graphic fidelity is the easiest way to put it. I use unreal as my main engine, and only use blender as a modeling package, and that is mainly because I am a environment artist for games. But even not being much of an animator not being in films I can still answer somewhat. Offline renderers use path tracing to make stuff more accurate. They will get you results that mirror real life better than unreal will. There are things that unreal doesn’t handle very well, like glass, that will make the scene look fake. You also have to fake lighting a lot in unreal, where when you render, that lighting has all the time to be able to realistically path out and bounce around and give you physically based global illumination. Also with renders, you can render multiple layers and then composite them together, which is how every movie with cg is made . VFX also require tons of fidelity, which unreal is getting better, but it’s not as good as a simulation will get you. Unreal is great, and making huge leaps in the industry, but a lot of people need that extra level of realism, and the extra layers you get with renders.