[taffer]

Also

[The relational model] provides a basis for a high level data language which will yield maximal independence between programs on the one hand and machine representation and organization of data on the other.

A further advantage of the relational view is that it forms a sound basis for treating derivability, redundancy, and consistency of relations [...]

So I would say it is mainly about flexibility, correctness and the possibility to create a simple yet powerful query language.