The style of parenthetical phrase is among frequently used discursive styles in the Holy Quran. In this research, with a review and detailed search in exegeses and literary books, the extensive use of parenthetical phrases in the Quran and their types have been reflected and then, after categorizing and differentiating single-purpose parenthetical phrases from multi-purpose ones, the overall purposes hidden in the verses have been identified, examined and documented. Next the four purposes of emphasis, prevention of crossing mind, expression and refuting the remarks of Kuffar (disbeleivers) have been considered to be the most used ones. The goal of this research is to answer the question why the style of parenthetical phrase is used in the Quran and resolve the ambiguity regarding a lack of proportion and relation between verses containing parenthetical phrase and other verses. This research is an analytical-descriptive one in which some 20 cases of purposes of parenthetical phrases are identified and by examining them, it is proved that the verbiage resulting from the style of parenthetical phrase in the Holy Quran is completely purposeful, fitting and appropriate, and in terms of quantity and quality, it is far beyond the styles used in Arabic poetry and prose texts. It is also one of the major factors of the Holy Quran’ s literary, rhetoric and content miracles.