The Next Best Thing?

why do we use the phrase: it's the next best thing since sliced bread? i mean, whoever thought of sliced bread didn't do anything special - he didn't invent bread OR the slicing method. all he did was put the two together!! what other technologies/investions can we put together to create something new, or something that is better than before? at the very least we've got to be able to come up with a better phrase than that.