What exactly is diffucult to understand about the value proposition? Seems pretty obvious to me...
If you mean why use dev environments in containers:
1) reproducibility,
2) re-use of container creation scripts for different environments,
3) isolation from your actual OS,
4) ability to run the same OS/libs/etc as the final deployment,
5) tons of base images with different environments already configured - from LAMP to data science,
6) easy sharing with others, team
7) ability to work with 1-2-5 or 100 different environments, with different OSes, versions, libs, python versions, whatever.
As for why have your IDE/editor work from inside a container (as described in the article).
Well, because you get all the benefits of containers (as above) PLUS get to use the editor as if you were programming directly on the target machine (including having visibility of installed libraries for autocomplete, running code directly there, and so on).
It's the same thing people have been doing with running Emacs in "server" mode inside another host, and programming with Emacs client on their machines as if they were locally at the machine.