Vast quantities of by-products generated during plant-based food processing operations represent a rich yet underutilized source of dietary fiber (DF) and bioactive compounds. These offer immense potential for gut health modulation and sustainable resource utilization. This comprehensive review systematically elucidates the classification, advanced extraction technologies, and diverse health-promoting mechanisms of DF derived from these plant-derived processing residues. A central focus of this work is its in-depth examination of the synergistic interactions between DF and polyphenols, uncovering their profound potential in enhancing bioavailability and biological activity—a perspective that contributes to a more comprehensive understanding of their combined impact on health. The review further delineates DF’s crucial role in regulating metabolic functions, including glucose homeostasis, lipid metabolism, and intestinal barrier integrity, primarily mediated through gut microbiota-derived short-chain fatty acids and immune-modulatory pathways. This review also explores the application of precision nutrition frameworks to develop individualized fiber interventions. Leveraging host genetics and microbiome variability, it proposes a refined paradigm for future personalized health management. The integration of advanced extraction technologies with detailed mechanistic insights into fiber-polyphenol interactions provides a robust foundation for the development of innovative functional foods and the high-value utilization of these plant-based processing by-products.