Because

The PURPOSE, not the mechanism — the sentence a catalogue shows and a docblock hides. On a class it explains why the operation exists; on a parameter it becomes that field's `description` in the derived input schema, which is the text an agent reads when it decides whether it has enough to act. Today that sentence is typed twice — once in prose for the human, once in a hand-written schema for the machine. Here it is typed once.

public function __construct(string $purpose):

Parameters