The Patriot Day movie is indeed based on true events. It recreates the incidents and emotions of that day with a fair amount of accuracy. However, for cinematic purposes, there might be some artistic liberties taken in the storytelling.
One key element is the remembrance of the victims. So many innocent lives were lost in the 9/11 attacks. Another is the heroism of the first responders. Firefighters, police officers, and EMTs risked their lives to save others. Also, it's about the unity of the nation. People from all walks of life came together in the face of this tragedy.