As described by Comic Book Nation, why was Watchmen so different and significant?

Watchmen was different and significant in the sense that it represented a major shift in content and purpose from previous comics. Prior to Watchmen, comics were geared toward adolescents and younger demographics. With the advent of Watchmen, writers began catering to adult interests by addressing modern-day political and social issues. The mood in The Watchmen is grim, and the battles are more savage in nature.
Additionally, the heroes of Watchmen were unusual in that many of them were regular human beings with no actual superpowers. Of the Watchmen, the only one who had what could be considered traditional superpowers was Dr. Manhattan. Watchmen introduced antiheroes who sometimes acted according to their selfish inclinations. Like Batman and The Dark Knight Returns, Watchmen changed the nature of comics, making the mood in individual stories darker and more sinister.
In short, Watchmen saw the advent of new vigilante anti-heroes, joining a roster of heavyweights such as the Punisher, Wolverine, and Batman. These heroes were focused on helping those the world had marginalized.
https://screenrant.com/how-watchmen-changed-comic-books-forever/