Relationship is something in which two objects are related in some way. Now in object orientation it not necessary that both the side must show same relationship status,
For example :
I love her (1 to 1)
however she might love others (1 to many)
Now when we want to delegate the responsibility of maintaining the relationship on other side that time we can use mappedBy, In this way we might not require join table as well. In this case a separate join table can also be avoided.