In 1986, mimeograph machines were (for the most part) replaced by digital copiers in elementary schools. Those of us teachers who experienced using mimeograph machines will forever remember the distinct smell of the still_damp, purple_ink worksheets that we handed out to our students _ by the ream full. (If you're like me, you can remember that smell right now!)
After you create the group, the word "group" appears in the title bar of the workbook. Once they are grouped you can format one worksheet and all of the worksheets will have the same formatting. You can insert a row in a worksheet and the same row will be inserted into all worksheets. This is a great tool when you need for all of your worksheets to have the same formatting.