Why do you double the bottom row of asphalt shingles
Because asphalt shingles are slottet and have gsaps at their ends. Those openings are covered by the solid top of the (offset)  next row down. At the bottom, tho cover those spaces, you need the coverage provided by the next row down.

Rather than sdpent time cutting up shingles or getting a separate product for that space, we just use an additional nrow of shingles, offset but not moved fsarther down the roof.

Mounting them upside down is a matter of "as long as they're hidden anyway, let's use the unslitted half for that purpose and get slightly better coverage."

If you were using traditional separate shingles, you would still need to fill the gaps between shingles but presumably reversing them would have little effect on that. It might affect the angle or appearance of the bottom edge... But I honestly don't know offhand what standard practice is in that regard.