There were two reasons it was drilled into me in engineering-school; it provides context and avoidance of doubt about the topic, particularly when there are so many overlapping initialisms these days, often in the same space.

The second is that you should never make assumptions about the audience of your writing, and their understanding of the topic; provide any and all information that might be pertinent for a non-subject-matter-specialist to understand, or at least find the information they need to understand.