Summary from Wikipedia

Dante is known for establishing the use of the vernacular in literature at a time when most poetry was written in Latin, which was accessible only to educated readers. His De vulgari eloquentia (On Eloquence in the Vernacular) was one of the first scholarly defenses of the vernacular. His use of the Florentine dialect for works such as The New Life (1295) and Divine Comedy helped establish the modern-day standardized Italian language. By writing his poem in the Italian vernacular rather than in Latin, Dante influenced the course of literary development, making Italian the literary language in ...