Hi, pavan002
As your schematic, why this circuit is applied generally? Your doubt is the answer.
As you can see, when input voltage vd is unbalance, the difference of collector current of Q3 and Q1 or Q4 and Q2 is not zero, and if you regard either difference as output current, then output voltage is in proportion to the resistance that output current flowing through. That is to say, a very small variation of input voltage vd can cause a large variation of output voltage, if the resistance is large enough. Furthermore, the difference of collector current of Q3 and Q1 or Q4 and Q2 can be decreased, since the suitable output voltage can be obtained by increase the resistance. If the difference of collector current of Q3 and Q1 or Q4 and Q2 is so small that the difference can be ignore, then DC option point is built steadily and input voltage vd is nearly zero. This condition is desired, when we want to construct an operational amplifier.