Put simply, the 'D' mode is for normal driving, and 'B' mode should be used when you are going downhill.
Is that a question or a statement?
I use B mode for urban driving because it's the closest I can get to engine braking in my previous manual gearbox cars. I like to look far ahead and coast toward red traffic lights and other obsticles so I can just continue when they clear. I try to minimise braking as much as possible; that practice is more comfortable for the occupants of the car, the car itself and fuel consumption. When I want to use Adaptive Cruise Control, which I do on fast roads, I have to use D. I just switch to B or D as required while I'm moving.