Standard SSI acts as a parent child relationship. The browser (child) requests a page from the Server (parent). The Server creates the requested pages and sends it back to the browser. As far as the browser is concerned, it is a standard HTML file. You can view the source, save it to your hard drive and edit it, but the relationship is unidirectional, you cannot automatically break the edited file back into the components used to create it.
The MySSI publish command emulates standard SSI. The process constructs each page from its components, but instead of sending them directly to a browser for display, they are written to the "Web" directory where your browser can be used to view and navigate through them. The content of Web directory can be published to your hosting server without the need for SSI / SSP support.
This is a nice feature, but it does not do very much for the creations and maintenance of a website. The next page will make those steps much easier.