When you want a copy of an email to be sent to someone, but do not want the primary recipients to know the email was also sent to the BCC'ed person.
Do not use if you would want the person in the BCC to be able to contribute in a response to the original email.
A good example and use would be if you were having a problem with a coworker and wanted to BCC the principal or HR so that they have a recorded copy of the conversation in question.
Use when you would like everyone to receive a notification of your response.
Do not use if there are people, possibly in CC field, that you would not like to see your response.
For example, a good use would be responding to an RSVP of whether you can make a work party or work meeting. You could use this in a parent email thread and have parents response with what they would each like to contribute to a class party. Reply all would allow everyone to bring something that they know no one else is bringing.