Isn't that like saying the purpose of a free government is the purpose of a free government?
It begs the question---what are the proper functions of a free government? One can't write a constitution without first answering that question.
I don't think Brutus was advocating that the language of the Constitution should give a government the power to "pass laws for the public good." Far from it. See his commentary on the preamble, for example.
The authors of a constitution can put whatever they want in the constitution.