The Primary Category plugin makes it easier to choose the main category for your post or product. Typically, when there are too many categories tagged in your post or product. One category is automatically chosen as the main category and used for the permalink and breadcrumbs. Adding additional categories may lead to changes in the permalink and breadcrumbs. This may cause a ‘404 Page Not Found’ error to appear, influencing your SERP/SEO ranking.
The plugin works by fixing the intended category in the permalink and breadcrumbs, thereby preventing such errors for occurring.