Lets say you needed to send an existing newsletter to one of your members/people.  You could do this by simply going to the newsletter and clicking on Send to Individual, then finding the person. But in this case, it's only sent to that person and you can't modify it.

If you want to modify the message or include other people in the e-mail, you can use the Send E-mail feature on the CRM. 

When clicking on Send E-mail, you can enter the recipients, have it come from your login e-mail, include CC and then click to Include an existing newsletter in the message.  This will then pull up a list of newsletters that you can include.

You can then edit the e-mail which is fully rendered for the recipient.  It takes on the same rules as the newsletter.