You must recieve some emails which you wanna convert into a documents for future reference or for printing as google docs providing you the ease of editing and creating documents without installing any  softwares.Now google has introduced one another very cool feature which allows you to easily convert any email into Google docs.After enabling this feature you will have option of create documents when you open email.

To turn on this feature, go to the Gmail Labs tab under Settings, select “Enable” next to “Create a document” and hit “Save Changes” at the bottom

Via : Gmail Blog