“It was traditional in the Renaissance to imagine that cuckolds—men whose wives committed adultery—had horns on their heads. Benedick's evocation of this image suggests that any woman he marries is sure to cheat on him.”- Sparknotes I would word it differently btw
I think the casket is a symbol of death and it is also a symbol of cruelty of the narrator that he took Doodle to see it and touch it to emphasize that they thought he was going to die so needed to be brought face to face with it.