As you have no doubt read, one uses amnesia when the interference of the conscious mind would interfere with needed changes. This circumstance can occur in a number of situations. For example, when aspects of the change are inconsistent with clients value system (and where other aspects of the necessary change are not); can also happen due to social factors such as a situation in which the social reinforcers could retrieve your or otherwise prompt undesirable when if a client were to choose to ak about it; and another popular reason includes helping the client to break the habit of attending and being distracted by a concern (especially one that has been changed by therapy). Of course these are the only reasons and both a complete list of ideas would be long enough to writ list of ideas would be long enough to write a book about. As for your idea that the conscious mind needs to be present to "work through" learning - it would be impossible to apply that idea to the cirumstances where the LEARNING is to not notice/pay attention to something.