THEORY: Why more than two grammatical relations?

(BTW:  Does anyone know which natlangs are thought to have four GRs, and
how accepted or controversial that analysis is?)

What advantage does it confer on a 'lang (whether nat- or con-) to have a
third or a fourth grammatical relation?

Lots of relatively recent discussion on this list and elsewhere has shown plenty
of use for at least one, i.e. a Subject; and in conlanging I have found it
convenient sometimes to have another, i.e. an Object.

But what use do natlangs -- or conlangs either for that matter -- make of
having two different kinds of Object, for instance Direct Object and Indirect
Object, or Primary Object and Secondary Object?

How does it serve the purposes of natlangs to have three grammatical
relations rather than just two?  How does it serve the purposes of conlangs --
your own, or others' you know well -- to have three GRs instead of just two?

AIUI languages with tritransitive verbs -- three kinds of Object -- are usually
thought of as having a Primary Object and two kinds of Secondary Object.
How does it serve a language's purposes to have four grammatical relations
rather than only three?

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