The A4 does use this technology, but Apple certainly did not invent it (though I wouldn't be surprised if they claimed so). It has been used in cell phones for years, and only recently others are starting to adopt it, and extend it to place two chips side by side on a Si substrate within a single package. The reasons are purely economical. In the past, much benefit can be had just by shrinking the transistor size. At the current feature sizes, though, physics is not helping anymore and there is very little benefit in terms of raw transistor and wire speeds, and power. So chip makers need to find more ways to extend Moore's law a few more years. FinFETs and stacked chips are just such ways.