Of all the construction trades that you're capable of doing, why did you choose plumbing? Where did you get the bulk of your plumbing knowledge?
Understanding how much you know and how you apply it is critical for any interviewer's decision making process. In their eyes, it's a major selling point that you have expertise and tested knowledge as a plumber and that you have the capacity to build and expand that expertise in the future. So before you respond to the question, think about why it's being asked. Is it to gauge your qualifications for the job? Is it to try to project what you might be doing in future roles? It's actually probably a little of both. Your goal in your response should be an enthusiastic overview of what you're bringing to the table and consequently the benefits of hiring you. Even if the interviewer isn't a plumbing expert, craft your response so that it's clear to them that what you do, you do well.