Why there is the need of detaching and merging entities in a ORM?

When should I Detaching an entity?
Detaching an entity from the an EM (EntityManager) is widely used when you deal with more than one EM and avoid concurrency conflicts, for example:

$user= $em->find('models\User', 1);
$user->setName('Foo');

// You can not remove this user, 
// because it still attached to the first Entity Manager
$em2->remove($user);
$em2->flush();

You can not take control of $user object by $em2 because its session belongs to $em that initially load the $user from database. Them how to solve the problem above? You need to detaching the object from the original $em first:

$user= $em->find('models\User', 1);
$user->setName('Foo');

$em->detach($user);
$em2->remove($user);
$em2->flush();

When should I use merging function?
Basically when you want to update an entity:

$user= $em->find('models\User', 1);
$user->setName('Foo');

$em->merge($user);
$em->flush();  

The EM will make a compare between the $user in database vs the $user in memory. Once the EM recognize the changed fields, it only updates them and keeps the old ones.

The flush method triggers a commit and the user name will updated in the database

You would need to detach an entity when dealing with issues of concurrency.

Suppose you are using an asynchronous API that makes callbacks to your project. When you issue the API call along with callback instruction, you might still be managing the entity that is affected by the callback, and therefore overwrite the changes made by the callback.