TAKEOFF MODE
With the airplane on the ground and the FMC indicates that ground
speed is >65 knots, the pressure controller enters takeoff mode and
the outflow valves move towards closed position to pressurize the
cabin to 50 feet below the takeoff field elevation at a nominal rate of
500 feet/minute, to minimize the effect of pressure bump at airplane
rotation i.e. improve passenger comfort.
The above is quoted from the 747-400 Engineering Training notes. I know it's a different type than you asked about but the reason for the initial pressurization is the same- i.e passenger comfort! JenCluse beat me to it - his statement is the exact reason. The outflow valves are at the rear of the fuse and the rotation of the aircraft at lift off moves the outflow valves 'down' and/or they descend in real terms - whilst at the same time the front of the aircraft (where the important people sit i.e first/business class/tech crew) climbs and/or ascends in real terms. This transient difference in altitude from the front to the back (combined with the actual increase in altitude of the aircraft as a whole) can't be catered for by the cabin pressure controllers. Thus to overcome this, the initial pressurization removes the pressure 'bump' that would otherwise be experienced with the accompanying discomfort to sensitive ears. The 737 doesn't quite have the advanced cabin press controllers and the interface to the FMC's of the later aircraft so this iniotial pressurization is accomplished as you say by selecting minus 200ft.