Disambiguating Databases

I disagree with the assertion that the main advantage of relational databases is to save disk space.

The main advantage of the relational model, as with any solid method of abstraction, is to save developer time and enable stronger use cases by providing a provably correct way of reasoning about data (predicate logic, relational calculus, etc.), which you do not get with NoSQL.

Unlike, say, functional programming, it's also quite easy to teach the relational model properly to complete beginners (like non-technical finance department staff) in a matter of days or weeks, which makes it a uniquely high ROI paradigm to add to your company.

I strongly recommend reading through at least the latest book by either Edgar Codd or Christopher Date.