It is to say that there is not only one truth but one way to teach it. It is important to realize the necessity of orthodoxy. Heresy is not a rejection or denial of the Faith. That would be unbelief or apostasy, and can be easily identified. Heresy will be found only in the professing church.
False doctrines can be far more pernicious than denied doctrines.
False doctrines added to the Faith can give new priorities, new emphases, and lead to new and false assumptions or conclusions. Ideas have consequences, and many of both are not doctrinally legitimate. The greatest problems Christianity has faced have been from within. Without we expect unbelief, denial, and even challenges. But wolves are never sheep.