As a teacher of IB Psychology for over 20 years I have marked hundreds of essays. The best essays explicitly state an argument in the introduction, develop that argument with reference to research in the body of the essay, and then reflect on the argument in the conclusion.
An argument gives an essay direction and coherence. Without an argument, the essay tends to be too descriptive and not analytical enough. There is little point in describing a study if it is not linked to an argument.
In terms of the IB criteria of the Extended Response Question, an argument helps a student show that can explain an issue and not just identify it (Criterion A). Criterion C is directly related to how the research selected by the student is effectively used to develop the argument. An argument also demonstrates well-developed critical thinking (Criterion D). Finally, an argument can demonstrate clarity and helps organise the essay (Criterion E).