There are many times when you want to have a backup copy of the web page you are working on. For example, if you are having problems with the content being shown improperly, you might want to change some of the code. But you want to have a copy of that web page in case you delete too much code or you delete the wrong code.
In Expression Web (and FrontPage), this is easily done by just highlighting the file, press CNTL-C (copy) and then CNTL-V (paste). This will create another file with _copy(1) in the file name. For example, if you copied default.html, the new copied file name would be default_copy(1).html. If you copy the file more than once, the number will increase: default_copy(2).html.
To see an example, you can watch a video tutorial of creating a backup copy of one web page.