If you look back on an email you've previously sent or receive an email from someone that you'd like to repurpose as a Template, there's no need to copy and paste! Simply open the email and click "Save as Template" located at the top of the email. Make any changes (like pulling out recipient-specific information) and save that Template for future use.
You can also save an email you're working on before you send it. Using the compose window, you’ll have access to Gmail's rich text features that let you change the font, font size, colors, and other general formatting. If you've written an email in the compose window that you'd like to save as a Template, simply click the "Templates" button at the bottom of the compose window and then select the "Save as Template" option. This will save a copy of this email to your Template library, allowing you to make edits and updates before saving.