Whatever you’re doing on your computer, you’re going to be using documents of all shapes and sizes. It’s very likely that a lot of these documents are very similar to each other. (Just think about how many emails we write each day.) Instead of starting from scratch each time, use a template so basic elements of your document are automatically created each time you need a new one. Mac OS X makes creating templates extremely simple by allowing almost any document to be marked “as stationery.”
Whenever you open a document marked this way, your Mac will open a copy of the document instead. I use this feature daily not only to create templates for things like letters and invoices but also to ensure consistency in my work.