This paper constitutes the fifth and concluding essay in the series What Is Philosophy?. The preceding essays have progressively explored whether philosophy possesses a shared fundamental foundation, reconsidered the relationship between love and wisdom, and proposed philosophy as humanity's highest integrative activity of wisdom. Building upon those discussions, the present paper turns to a further question: What is the function of philosophy within human civilization? Rather than treating philosophy primarily as an academic discipline or a collection of competing schools of thought, this paper argues that philosophy performs an enduring civilizational function. It continually returns to humanity's most fundamental questions, reflects upon inherited intellectual traditions, responds to the defining challenges of each historical age, and participates in shaping the future through the philosophical orientations civilizations choose to embrace. The paper further argues that many contemporary difficulties arise not from the disappearance of philosophy, but from the fragmentation of its integrative function. Recovering philosophy therefore requires restoring its proper place within civilization, clarifying the distinct functions of different forms of philosophical inquiry, and reconnecting philosophical reflection with human practice. The study concludes that philosophy remains indispensable not because it provides definitive answers, but because it enables humanity to understand itself, guide the development of civilization, and participate more wisely in shaping the future. This paper completes the What Is Philosophy? series and serves as a foundation for a forthcoming book that integrates all five essays into a comprehensive reconsideration of philosophy's nature, function, and civilizational significance. Keywords Philosophy; What Is Philosophy; Philosophy of Civilization; Wisdom; Civilizational Development; Shared Philosophical Foundation; Integrative Wisdom; Philosophy and Practice; Ontological Philosophy; AI Ethics; Human Civilization; Future Studies