In SharePoint, most (not all) files don’t actually exist in the conventional way. What I mean by this is that you can’t just browse to a directory and open a SharePoint page in your favorite text editor and make changes. Instead, most files in SharePoint exist as BLOB data in a SQL database and the only way to actually edit these files is through SharePoint Designer.
So what if you want to have any of these files in source control? Of course you could use SharePoint Designer to copy/paste the contents of each target file to your file system, but that is extremely cumbersome (you can’t just copy/paste a file itself, you need to actually open it and copy the contents). As our clients have become more and more proficient with using SharePoint Designer to make their own changes we have had an increasing need to actually get these changes in source control.