Well, the multipass render setting saves parts of the render out as seperate pictures or movies. The interesting thing about the multipass settings is that you can combine the different pictures into one Photoshop file with different layers with the correct transfer options.
AE export adds a bit to that. First of all, it gives you a project file that combines your movie layers in the correct way (correct layer fransfer). Second, the AE export also transfers the camera position movement and the light settings from your scene. This allows you to blend a 3D layer in AE without any glitch in your rendered movie(s). There’s a couple of other options too (more exotic), but that’s more or less the bottomline.
I guess the Combustion export does about the same. We already had RLA and RPF export, but I don’t know how good it was and how it was treated in Combustion.