B737NG_Pilot - the only reason for is to prevent the VNAV from going into a clacker.
Unfortunately incorrect.
VNAV will prevent the aircraft flying into the clacker as it is restricted to KIAS 330, Mach .81
In VNAV PATH, it will dive to try regain the path and increase the speed doing so (crew could assist VNAV by reducing the speed increase using the speed brake of course).
When reaching KIAS 330 the aircraft will forego on its effort to regain the path by not further increasing the speed but by maintaining the maximum of KIAS 330 or M.81, until the calculated PATH is regained and VNAV PATH resumes.
Should at ANY stage a VNAV SPD/LIM be triggered such as e.g. KIAS240/FL100 then it will level off at that level, wait until the speed is within the restricted limits (in this case 240 +10) then drop the nose again and obey the new restricted speed
+10 knots.
The message UNABLE NEXT ALTITUDE will be displayed when VNAV is then unable to cross subsequent altitude restrictions coded into the FMC.
VNAV PATH is very clever, and does what it is supposed to do: PROTECT THE AIRCRAFT AND ITS CREW from an overspeed (M.81 and KIAS 330, or coded speed restriction +10 its) or an under speed (-15 kts or lower band limits).
In VNAV SPD however, any descent speed restrictions are ignored if you are controlling the speed using SPD INTV with an open speed window on the MCP, a practise regularly observed on the line. VNAV SPD will obey limitations if selected through the FMC on DESC page or VNAV page 3.
It is a very common misunderstanding and incorrectly taught by unknowledgeable trainers (to mostly new cadets) of how VNAV (PATH) works when saying it will go into the clacker.
Hence my original posting (22/12/13) asking for the FMC version used, to determine the small ambiguities and changes as time has progressed in VNAV-land.