This article explains how to copy a text style in macOS. If you don’t copy and paste the text style, you’ll just copy the text. You can end up with different styles and formatting in the same email, which usually doesn’t look good.

To copy a text style and apply it to other text while writing an email message:

Place the cursor in the text with the formatting you want to copy.

From the menu, go to Format > Style > Copy Style.