Here's a scenario you may relate to: Imagine that you use a certain document, an invoice, over and over, to bill a client. Its basic content stays the same, but certain details change, such as the type of work done and the dates involved. Your method is to open the invoice document, change those parts that are outdated, and then save the document with a new name.
In effect, you're using your original document as a template, although you may never have turned it into one. Why not do so? By turning your invoice document into a template, you will have one clear master to work from, and you won't have to undo outdated content when you're preparing a current invoice.
You can beef up the template with everything you always want in the invoice, including text placeholders that make typing in new stuff easy. Then always use that one template file as the basis for all your new invoices.
There are more advantages. See what they are. Then learn to save a file as a template, and how to find, use, and edit it.