Greatest Thing Since Sliced Bread Meaning

Example : 1962, Jeanne Boardman, letter to "Hints from Heloise", St. Joseph, Missouri, Gazette (Wednesday, December 19, 1962), page 5:  This column is about the greatest thing since sliced bread.1978, Joe Girard, Stanley H. Brown, How to Sell Anything to Anybody, page 104:  Because if he starts to write a check for the full amount and I say, "Take off $250", or "I'll throw in door-guard trim and five steel radials for nothing", Charley is going to think I am the greatest thing since sliced bread.2007, David S. Platt, Why Software Sucks: ...and What You Can Do About It, page 208:  It was the greatest thing since sliced bread when it first came out, and I remember selling a big project to a customer by demonstrating it.