As far as analogies go, is pretty far off. It doesn't hold even for basic behaviors of the two cases, let alone complex ones. A better analogy would be that you buy a (small) car that always happens to come with an included, free trailer for more cargo capacity. You can of course take it off and have a small car. And it's also as magic trailer that doesn't take up any space at all when not in use, but can also not be sold.

I'm actually not a fan, an am also using it somewhat reluctantly personally, though self hosted. I've had my issues with it, but an still using it because it solves some issues that are much harder to solve without it. I'm not using the contacts/calendar functionality.

But your original statement was that you couldn't understand who would need calendar and contacts (in their file sharing app). There's enough I object to in this statement that I wrote my comment. First of, in this context, specifically in their article/blog/whatever, it's about nextcloud as a whole, not the fact that it can do file sharing. That's what it evolved from, but not all it is any more, more better or worse. Secondly, it's about an advertised alternative to O365, which includes the very common and almost universal requirement for teams (be it a company, family, ...) to have events (=calendar) like schedule meetings with people (=contacts). Even if you work with just like 5 people you are probably gonna need that. There you probably want to share files, but probably more so it's about the office functionality and collaborative, simultaneous editing of files. Obviously replacing Word, PowerPoint, Excel. And yes, Outlook (calendar,& contacts, also email).

This isn't meant for individuals who need a few GB to store some files. It's for teams of some description that need office like, cloud based tools.