Please Explain Weld Thru Primer

I’ve been told that it’s intended use if for doing metal work where a lap joint is involved. Let’s say you are welding in some floor pans, since these panels are quite large it can be tricky to trim them in such a way that you can butt weld them. So a lot of guys will usually overlap the two pieces. But before welding it the weld thru primer is supposed to be applied to the area that Is overlapped. Unlike a butt weld, moisture may find its way into an overlap joint and create problems I’ve heard that the weld thru primer Is a way to prevent this type of problem.