The starch-binding domains (SBDs) are structurally and functionally independent non-catalytic protein module widely found in carbohydrate active enzymes (CAZymes), and they play an important role in the degradation process of raw starch in ecological environment. In nature, SBDs effectively lead the catalytic domain to the substrate surface by specifically binding to starch granules or other polysaccharides, significantly enhancing the hydrolytic efficiency of enzymes. In addition to the non-hydrolytic disruptive activity on the structure of starch granules, SBDs widely influence many catalytic properties of host enzymes, including substrate specificity, environmental adaptability (thermostability, salt tolerance, and cold adaptation), and oligomerization state. These roles demonstrate their adaptive function in helping host microorganisms to compete for carbon sources in complex habitats. Here, we summarized the distribution characteristics, functional mechanisms, and multiple effects of SBDs on host enzyme functions in terrestrial and marine ecosystems. The main experimental methods for studying SBD functions, including domain deletion, functional characterization of binding and disruption, are also introduced. A broader and deeper understanding of SBDs enhances our knowledge of microbial competition for carbon sources in natural environments, as well as provides novel approaches for the rational design of industrial enzymes.