OneToMany composition is useful when you wish for TransferObjects on both sides of the relationship to see each other, or for the Parent to have a collection of the child objects attached to it.
A ManyToOne collection is useful when, either for application design, or performance reasons, you only want an Objects to load one side of the relationship, and not generate a collection of Objects.
For example, if I have a series of Comments on a Blog Post, I want the Post to have a collection of Comments on it, so I would use a OneToMany.
However, if I had a Product with which I could generate hundreds of Orders, I would build my Order object with a ManyToOne to my Product, so that a Order would have a Product attached, and the Product would have no need to be aware of it's Orders.
In other words, you can think of OneToMany as OneWithMany, and ManyToOne as one of ManyWithOne.