Well, that's false. The actual US Constitution in Article I Section 8 Clause 8 says, "[The Congress shall have power] To promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries."
That could, possibly one-day provide public benefits, but it doesn't have to. If public benefits happen, they are side-effects. The purpose is for authors and inventors to have exclusive rights in their writings and discoveries. They are never required to publish or advance anything for the public. This is a private protection not a public benefit for a "specific purpose".