Full Idea
Model theory must choose the denotations of the primitives so that all of a group of sentences come out true, so we need a theory of how the truth value of a sentence depends on the denotation of its primitive nonlogical parts, which Tarski gives us.
Gist of Idea
Tarski gives us the account of truth needed to build a group of true sentences in a model
Source
Hartry Field (Tarski's Theory of Truth [1972], §1)
Book Reference
'The Nature of Truth', ed/tr. Lynch, Michael P. [MIT 2001], p.369