This is very helpful in situations where you want to take a big block of code and repeat it throughout a series of pages. For instance, maybe you have a ten page site where you want to have the same set of buttons appear at the top of all pages. You could hard code the buttonbar code block into each page, or alternately you could hardcode the buttonbar code block into its own little document, and then include that document in the ten pages that use it. Doing it the second way is not only less work, but it has the additional advantage of if you want to change the buttons at a later time, you only have to change them in that one document and the change will ripple through the other ten automatically.
Server side includes are very easy.
Where myfile is the name of the file you want to include. Otherwise, the server will not recognize the include line at all and will write out the page as if the include is not even there. Here's another example using the buttonbar idea expressed above. This document serves as our button bar, wherein we have four graphical buttons that point to index.shtml, products.shtml, support.shtml, and contact.shtml respectively. Once you do this, all pages will contain the button bar.