Greg Balint said it all very eloquently. One thing I will emphasize is the seamless back and forth of being able to bring C4D elements like lights and cameras into AE and vice versa. I have to get more into it, but being able to tweak multipass render layers on the fly is also incredible. I wish I had had these tools five years ago! Cameras especially.
I also do a lot of type animation and clients are always asking for 3D type. While previously possible, that is something that will benefit enormously from global illumination and the other FULL 3D render features of C4D.
Additionally, I hate building 3D scenes and working with many 3d elements in AE. Even with the raytracing engine, or Element 3D, the viewports have always felt very cramped. It's like constructing a ship in a bottle with the wrong prescription eyeglasses. With a proper 3D environment you can see everything going on outside the frame and manipulate objects 2x as fast. Even without using ANY of the render features of C4D, being able to do huge 3D multi-plane comp fly-throughs just got so much easier. And infinitely more interesting. Imagine taking a pre-built AE comp fly through and being able to cleanly add mograph effectors to every layer, or using cloner objects as video screens etc. Like Greg said it's a huge boost to what's possible in a normal working environment with production deadlines.