Subsites are an important concept for site management. Expression Web treats subsites as though they were completely independent sites.
One reason that you might want to segregate certain folders in a site as subsites is that you might have a large number of files within those folders, and the files don’t change as often as the other files in the root of the site. There’s no reason to wait out a long file-comparison cycle during an Expression Web publishing session when you know that the files haven’t changed.
Another good reason to use subsites is that you might be developing a replacement site. You can create a subsite on your server-based site, such as ExpressionWebStepbyStep.com/new-design/, and then publish your new design directly ...