Features and CMI Together
In my opinion Features isn't really useful for just one site. It is useful for extracting and packaging functionality to use on multiple sites. Using both at the same time is probably counter productive and prone to errors, as they both try to enforce features via configuration and manage them as well.

If you had a site and wanted to copy/clone the article functionality or product catalog functionality from it to another (different) site, then, yes. You would need Features to do this, because core CMI does not work when you want to take pieces and import to a totally different site.

On the other end of the spectrum, you could ignore the core config management and use Features to do this for your entire site, but it is more for you to manage, and as stated, if you are not extracting it out for use in other sites, it is just busy work.