Best thing since sliced bread? What does it mean?

Factual Questions
1

I know how that expresion is used, but what is so special about sliced bread?

2

Only that commercially pre-sliced bread didn’t exist before 1928, when a bread slicing machine invented by a guy named Otto Rohwedder came into use, together with the idea of machine wrapping the loaf so it wouldn’t go stale. A lot of people probably found it convenient, and said “why didn’t anybody think of this before”:

http://www.foodreference.com/html/fslicedbread.html

As to why the saying has survived an era when pre-sliced bread was a clever thing people hadn’t seen before, I don’t know.

3

Sliced bread is instantly usable;ergo, more convenient. No looking for a knife to slice through a baked loaf, or a board on which to place it. No crumbs to sweep away. AND a wrapper to keep it tidy and fresh. What more could you want? Well,I guess some PB&J to smear on it!