What are the practical uses for relational calculus?

In my Database Design course we are learning both Relational Algebra and Relational Calculus. I can see where Relational Algebra could be useful since it is closely tied to SQL. Our professor said that Relational Calculus was used as an alternative to SQL in some RDMBS, most of which are not around any more. Is there still a practical use for Relational Calculus, or is much of this theoretical?