What is the use of NOT CONNECTED pin in 741 opamp?
The reason for no connect pins is that the manufacturers use standard ic packages.

The package for a given device must have (clearly) at least the number of pins required to bring out all functional pins; in the case of the 741 (and countless other devices) the number of pins required for functionality is less than the number of pins on the package.

Those pins that are not required for functionality are still required for the standard package and are simply not connected to anything.

On complex devices with hundreds (or more) pins, it is not uncommon to see numerous NC pins.

Updated for a comment by Spehro: I should have noted this, especially as I have recently been using a device that has just such an arrangement:


  The LT3752/LT3752-1 are available in a 38-lead plastic TSSOP  package  with  missing  pins  for  high  voltage spacings.