In chapter one of his book on Paul and the New Perspective, Stephen Westerholm argues the case for Augustine being included in the debate over the right interpretation of the apostle's writings:
Augustine had his own spectacles, to be sure, furnished in part by his struggles with heretics. The latter, like other perversions of the good in Augustine's universe, inevitably served useful purposes: they compelled the church to "investigate [its articles of faith] more accurately, to understand them more clearly, and to proclaim them more earnestly" (De civ. Dei 16.2.).
Westerholm, Perspectives Old and New on Paul: The "Lutheran" Paul and His Critics, p.3
Heretics are useful because:
1. They compel the Church to Investigate the faith more accurately
2. They compel the Church to Understand the faith more clearly
3. They compel the Church to Proclaim the faith more earnestly