Rule Intro
Sometimes, in order to add someone in to the loop on something, you might write an email and attach another email as reference material.
Generally this is not a good idea and you are better off replying to the original email instead so you can keep it in the history of ongoing threads.
Page Content
The exception is when you are already in a conversation thread with important history and you want to add more reference material to the email, although even then it's a good idea to also quote the particular line of interest as well.