Model theory is typically based on a formal language whose production rules and syntax are designed to formalize the deductive process, and as such typically include predicate or relation symbols. Obviously, any predicate by itself constitues a (atomic) formula, but any formula, when interpreted in a model, corresponds to a predicate as well, insofar as the formula corresponds to a definable subset of the model's universe which one could then define as the interpretation of a predicate symbol.
Neither of those correspondences are bijections, however though evidently predicates are extraneous, so the question is - are they blanketed in the language for the identical cause that the established quantifier is included no matter being able to be described using handiest the existential quantifier and negation image, i.e. for comfort? Or is the inclusion of predicate symbols genuinely important, either in version concept particularly or in the philosophical motivation as a rigorous examine of the deductive procedure?