I think originally notebooks were packaged entirely under IPython. But recently the notebook and language-agnostic parts were spin-off under the Jupyter project, while IPython still provides the Python shell and kernel.
At a more basic level, these notebooks enable live code in your documentation. At first I was wondering if it could be used as a learning tool, but the interface seems too clunky for that - I found it easier to just bang stuff out in a repl or actual code editor. But I can see the value of notebooks as a companion tool for scientific and engineering work. I know some people in quantitative finance who use it precisely for documentation purposes for their machine learning and algorithmic stuff.
For web dev, I dunno…
