Cultures are defined by their rituals. Some rituals are ubiquitous across cultures, such as marriage and holidays, whereas others differ across cultures, such as worship rites, rites of passage, oaths of allegiance, and the myriad rituals around birth and death. These rituals also vary in their expressive elements, ranging from hazing, self-mutilation, and animal sacrifice, to unique gestures, dancing, and food. Ritual is what brings us closer to our Gods and to each other, and perhaps what comforts us most in times of grief.
And religious zealotry is a mental immaturity/deficiency, yes.