You can click Save as Template at the bottom right of the screen to save your work on the offer letter as a template for future use, or to update the content of an existing template.
Saving a template is independent of saving a draft. If you save a draft, you're saving your work for the particular offer, whereas if you save a template, you're saving your work for use in future offers.
For example, you typically use the Save Draft button to save your progress as you work on the offer letter over a set period. Once you've completed the offer letter, you can use the Save as Template button to save a version of the offer as a template for use in future offers.