One side annotated with OneToOne and the other with OneToMany annotation

Question

At first I thought it was a mistake but then I found this very odd situation in several other tutorials on the net. I don't understand how this works.

User bean:

@Entity  
@Table(name="users")  
public class User {  

@Id  
@GeneratedValue  
private Integer id;  

private String login;  

private String password;  

@OneToOne(cascade=CascadeType.ALL)  
@JoinTable(name="user_roles",  
    joinColumns = {@JoinColumn(name="user_id", referencedColumnName="id")},  
    inverseJoinColumns = {@JoinColumn(name="role_id", referencedColumnName="id")}  
)  
private Role role; 

Role bean:

@Entity  
@Table(name="roles")  
public class Role { 

@Id  
@GeneratedValue  
private Integer id;  

private String role;  

@OneToMany(cascade=CascadeType.ALL)  
@JoinTable(name="user_roles",   
    joinColumns = {@JoinColumn(name="role_id", referencedColumnName="id")},  
    inverseJoinColumns = {@JoinColumn(name="user_id", referencedColumnName="id")}  
)  
private Set<user> userRoles; 

Can someone please explain how this works? And why do it like this?