> the ability to request operations against those relations declaratively That is true but I think the main significance is not that you can "request operation

> the ability to request operations against those relations declaratively That is true but I think the main significance is not that you can "request operations ... declaratively" but that the results of those operations have the SAME FORM as the thing/relation/table they were performed on. That is what makes relational algebra as a concept so powerful. It is like numbers. You can perform operations on numbers and you get ... numbers. You can perform operations on tables (a.k.a "relations") and you get ... tables.