IMRaD comes from the first letter of the most important sections in a research paper: Introduction, Method, Results and Discussion. Some research areas are more strict than others about these four sections being present, due to tradition and whatnot. However, by separating the material into these four major groups, you at least have a decent starting point.
Most of the material here is based on the course English For Research Writing, 4EN602, at Linnéuniversitetet, and the book Academic Writing: University Course, by Lennart Björk and Christine Räisänen. Together they provide a deep and comprehensive guide for making research papers easier to read. This site can then be viewed as a summary of my course notes, together with various opinions on what I personally like and don’t like to see.
Can a paper be accepted even if written in an entirely different way? Of course! But, as Magdalena Ribbing used to say: you need to know the rules, in order to know when and how to break them.
Will a paper written with this structure always be accepted? Of course not, you still need a proper research content. You will however make it easy for the reviewers, presenting things in the expected order.