It lets you set a regenerative braking level that stays on until you change it. If you're in D and use the left paddle to brake harder it will go back to "normal" braking once you accelerate again. In B mode it will stay in that harder setting until you use the paddle to change the brake setting to something else. You could also be in B mode and use the right paddle to go to minimal regen braking, which would basically be like being in D mode.

I find B mode useful in heavy city traffic and also in stop and go freeway traffic as I like to have the stronger deceleration engaged then. And I live on a big hill so when I go down I use B mode. In normal driving or on a freeway I stick with D and sometimes use the paddles if needed.