The markdown is good for describing what is going on. If you look at some of the example notebooks, they do a good show showing how it can help.

For example, this simple notebook uses it to give an overview of each code block and what's going on. Most of the time when using Notebooks you are dealing with a lot of data.

Another great example:

https://www.kaggle.com/stkbailey/teaching-notebook-for-total-imaging-newbies