Shift the weight to the backplate is more effective than just putting weight on a cam strap due to the face the weight is evenly distributed across the back between your lungs and the wing (your two sources of lift). That distribution gives the diver a great start in their quest for good trim.
Agreed, it's much nicer to dive when some of the weight is distributed next to your back rather than out on a cam band.
The bp/w offers more options for weight distribution. For example I dove my single rig yesterday, and experimented with additional weight plates that are screw onto the SS back plate (which allowed me to unload some of the weight from my weight belt).
Somehow the weight just seems lighter when distributed on the bp and the bp/w setup certainly helps with good trim. My instructor says that removing some of the weight from your weight belt helps gas efficiently migrate within the drysuit. But as others have said, it's important to leave enough weight on the belt - so that if needed you can ditch and have the strength to kick up to the surface.
Streamline is another magical function about the bp. Less bulk = less resistance in the water. This is especially true if you follow configuration to efficiently stow hoses, lights, spg, etc.