Emails that were repeatedly replied to and forwarded can become too long and heavy. It becomes a nightmare to sort through such long text to find relevant information. It is polite and useful to edit such emails when replying or forwarding further to make sure only the important part is being sent.

You can achieve this really easily by making a selection in the email you are reading before hitting the reply or forward button. By doing this you will only include the selected text in the newly created email.