Is the 33 ohm resistor sitting between the signal lines of the circuits and the rest of the network (ie. between the device and before the junction where the lines split up to other devices?) If it is, then it's probably for protection to try and supress current spikes that may be induced on the line.
It's probably 33 ohms so it provides enough current limiting against spikes or transients but is low enough to allow the maintain the integrity of the square wave's rise and fall times.
You don't need those resistors there so it's probably for protection.