Why is 50 hz 690V preferred over 440V in electric systems . Is there any benefits for 690V over 440V or vice versa . please answer?

At 690V the currents are smaller resulting in smaller rated equipment (smaller cables, Transformers, switchboards, equipment ..etc)

at 690V the running current is reduced by40% compared to the 400V system

and the energy losses due to hear are much smaller compared to the 400V system.

many motors are on the border line for a service voltage of 400V but they can easily be accommodated at a service voltage of 690/400V.

refer to IEC 60038 edition 6.2 2002-07.