The advantage is that the include can have body code, header css, and server side code to make the include into a functional module... it can be in a sub-directory, and all of the links can be relative to the plugin page and will automatically be updated to work when placed on a page in a different directory.
Make sure you move any associated css and server side code that goes with the functionality you moved into the plugin so you can keep it all together. That is the benefit of plugins over regular includes:
1) It allows you to keep your html, header, and server code all together.
2) Allows you to use relative links from sub-directories and updates them to work whenever and wherever the plugin is used.
3) Because the links can stay relative to the plugin location and not the page that uses them, all Dreamweaver functionality can be used without adjusting manually to reflect the end location.