You can use Dreamweaver to insert server-side includes in your pages, edit the includes, or preview pages containing includes.
A server-side include is a file that the server incorporates into your document when a browser requests your document from the server.
When a visitor’s browser requests the document that contains the include instruction, your server processes the include instruction and creates a new document in which the include instruction is replaced by the contents of the included file. The server then sends this new document to the visitor’s browser. When you open a local document directly in a browser, however, there’s no server to process the include instructions in that document, so the browser opens the document without processing those instructions, and the file that’s supposed to be included doesn’t appear in the browser. Thus, it can be difficult, without using Dreamweaver, to look at local files and see them as they’ll appear to visitors after you’ve put them on the server.
With Dreamweaver you can preview documents just as they’ll appear after they’re on the server, both in the Design view and when you use the Preview in Browser feature. To do so, however, you must make sure you are previewing the file that contains the include as a temporary file. (Select Edit > Preferences, select the Preview in Browser category, and make sure the Preview using temporary file option is selected.)