The simple answer is that the sensors are used to shut the engines down
before fuel exhaustion. The Space Shuttle Main Engine uses turbopumps
(that also burn liquid hydrogen and liquid oxygen) to pump sufficient
quantities of propellents to make it all go. The issue is that
turbopumps behave very badly, even destroying themselves if they are
run try, especially if at full power. It is apparently the nature of
the beasts. The sensors cue systems that begin to throttle the engines
back to about 65% power at which point the SSME can be shut down safely
and without damage.

Consider visiting the Science Space Shuttle News Group. Several people
asked the same question and the issue gets explained pretty clearly.
Someone noted that their car's fuel pump had a similar feature so to
protect it from burning itself out from running dry. Just like us, the
astronauts do not have the option of pulling the silly thing over to
the side of the road if something important decides to break.

blue skies to you all

John