Why is the inclusion in poetry effective?Referring to conflict.
Conflict helps to bring out true meaning and understanding in a character. As the reader, we only get to fully understand and grasp a character's function and purpose when we are able to see their character revealed through conflict. Examples in literature can prove this. For example, John Proctor in Miller's work, "The Crucible," might represent this quite powerfully. It is only through social and personal conflict that we understand his depth and through which he becomes a figure of mythic moral and ethical proportions. While the conflict might be agonizing, it reveals much about the individual and allows to fully absorb their suffering and pain. It brings us closer to the character. Hector from "The Iliad" is another example. It is moments such as the conflict he feels to wish to stay with his wife and child and the duty he feels to fight Achilles, from which he knows he will lose, that garners the greatest amount of empathy from the reader. This moment is one wrought with conflict and one that allows us to better understand what it means to be Hector and brings us closer to him.
Your question is a bit vague - but I'll answer it this way. Without conflict, a story does not exist. Consider the four modes of writing: narrative, descriptive, expository and persuasive. The only one of these that tells a story is narrative. The other three are just essays - pontifications if you will - on some subject. And these are not necessarily for the purpose of entertaining. More often than not, they aren't entertaining.
But narrative. Narratives entertain. Why is conflict necessary for a narrative? Well, conflict is interesting period. The entire premisis of what we humans enjoy about stories is the big problem, was it solved, and how? This is likely because we all experience problems on a daily basis. And in our daily lives, aren't these what make existance interesting? Reading about other people's problems is always fun because they aren't ours - and then we make comparisons of ourselves to others and enjoy thinking, "Well, my life could be worse."