From this link:
Can anyone explain why the Boeing 737 has a pitot tube for the elevators? Wouldn't the airspeed be measured the same from the front of the airplane and the tail? Is the airspeed measured at the tail view-able from the cockpit?
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There are five sets of pitot tubes on the 737, organized into two groups, the pitot tubes on the nose are used for airspeed measurements, independent for the pilot and copilot and one as a backup. There are two pitot tubes on the tail that are for the "elevator feel and centering unit" (pdf; page 8).
The elevator feel computer provides simulated aerodynamic forces using airspeed (from the elevator pitot system) and stabilizer position. Feel is transmitted to the control columns by the elevator feel and centering unit.
The airspeed may not be the same due to shielding in high angle-of-attack situations. I do not believe that the airspeed from the pitot tubes in the tail is visible (or useful) to the pilots. There is a warning that appears in the cockpit when the sensor fails.
The 2 pitot tubes on the dorsal fin provide pitot pressure (raw airspeed data) to the elevator feel and centering unit to provide artificial feel to the control column. The faster the aircraft flys the more force it will take to move the elevator fore and aft because at higher airspeeds less control inputs are required. This force (artificial feel) helps prevent the pilot from over controlling the aircraft at higher speeds. There are two because one provides input to the "A" side and the other for "B" side. The elevator feel computer (it's a mechanical computer NOT an electrical one) compares the output hydraulic pressures and if there is a difference it will illuminate a light on the P5.