If there were no constraints on possible use, then every argument would be invalid. Hence, the need to develop some constraints. Consider how we discover the truth-values of sentences in which words are used normally. So, the syntactic classification of a term fixes a broad range of possible uses for it due to the fact that syntactic elements have more or less well-defined semantic roles. This range is narrowed by the specific conventions of English and dictionary definitions.
Formal logic takes structural constraints as basic in characterizing possible uses.
A possible use will be any coordinated use of the elements of the sentences constituting an argument that is consistent with their structure. In terms of such a picture, an argument is valid if there is no possible use of the elements of the sentences of which it is comprised for which, in accord with their structure, the premises are true and the conclusion is false. If so, then the argument is invalid, the use constituting a counterexample in which the premises are true and the conclusion false. If not, the argument is valid. Obviously, this makes validity relative to the structures we are prepared to see in sentences.