http://www.tvtechnology.com/article/72382

Probably for small handheld displays like those on fancy cell phones. If it works, the reception would be more robust than regular ATSC set top boxes, and hopefully future full size DTV sets could use this new mobile/ handheld signal as a fallback if full HD reception goes bad. Assuming the program is a simulcast of the regular HD program.

"Central to the M/H system are additions to the physical layer of the ATSC transmission
system that are easily decodable under high Doppler rate conditions. Extra training sequences
and forward error correction (FEC) are added to assist reception of the enhanced stream(s).
Consideration has also been given to the many system details that make such a signal compatible
with legacy ATSC receivers, particularly audio decoder buffer constraints; but also such
constraints as MPEG transport packet header standards, requirements for legacy PSIP carriage,
etc. These changes do not alter the emitted spectral characteristics."

-- page 12 of http://www.atsc.org/standards/cs_documents/a153/S4-130r12-A153-Part-1-ATSC-M-H.pdf