SSI is very useful if you're building a big site where you want the header & footer for each page to be the same.
So if you were to change something in header or footer, you only change it in one page and effects take instant effect on all pages.
SSI commands will only work if your page has a .shtml extension.Eg if you were to create a page index.shtml and include this line inside it:
<!--#include virtual="/header.htm" -->
Then index.shtml will show the contents of header.htm.
So SSI is indeed very powerful, useful and time-saving for you.