Adjusting the offset of the output of a UA741CN opamp
What is &quot;offset null&quot; in IC 741?  has some explanations

The 741 is an OLD design and one often needs to tweak out the DC offset that exists on its output.  A pot between pin 1 and pin 5 with its wiper to -VCC (pin 4) is typical.  Your circuit is using fixed resistors.. don't know why.

The diode seems extraneous to the offset function.  Are you sure that's why it's there?