a) Just like with a paper, you can present scientific or mathematical ideas with accompanying visualizations or simulations. From the REPL side, as a bonus, you get interactivity, and the reader can pause and experiment with the examples you're giving to improve their understanding or test their hypotheses. If I change this variable, how will the system react? You can just try it!
b) Just like with a REPL, you can type in and execute commands step by step, viewing the output of the previous command instead of running the whole thing at once. From the document side, as a bonus, you get nicer presentation (charts, interactivity, nice and wide sortable tables, etc) than you would in a shell, which comes in handy when doing things like data exploration or mathematical simulation.
It's decidedly NOT there for you to type all your code in like an editor and make a huge mess. It's apples and oranges w.r.t and a poor substitute for something like PyCharm or VS Code or vim. It is there for you to a) try things out yourself, and whatever you discover hopefully eventually make it into proper python modules b) make interesting ideas presentable and explorable for others. That's all!
When I see stuff like "out of order execution is confusing", I don't disagree, but it does make me wonder how long and convoluted the notebooks these people work with are - probably a ripe candidate to refactor stuff out into python modules as functions. When I see stuff around notebooks for "reproducibility", I'm a bit confused in that notebooks often don't specify any guidance on installation and dependencies, let alone things like arguments and options that a regular old script would. In that regard I think it's barely an improvement over .py files lying around. When I hear "how do I import a notebook like a python module", I'm very very scared.
Granted, I've seen huge notebooks that are a mess, so I understand the frustration, but it's not like we all haven't seen the single file of code with 5000 lines and 10 nested layers of conditionals at some point in our lives.