Thread:Talk:BeagleBone Black Extracting eMMC contents/Rev. C boards from Element14/reply (2)

I think in this case you are using the S2 button to boot completely off the SD card and thereby getting a consistent U-Boot image. I'm just sticking the card in an powering up the board, which relies on the U-Boot image loaded on the eMMC. There's not a strong reason to do that except that booting without S2 is simpler and does not require using the barrel jack.