This is diagram of TDS sensor , i want to know why op-amp a is used as attenuator?
What can be the possible benefit?

I am using this sensor.
https://www.dfrobot.com/wiki/index.php/Gravity:_Analog_TDS_Sensor_/_Meter_For_Arduino_SKU:_SEN0244

 

It provides a buffered output with a lower output impedance than would a simple resistive divider comprising R2 and R4.
The probes probably use a low amplitude energisation to minimise electrolytic corrosion.

 

Does These full wave percision rectifier shown in circuit diagram provide voltage gain?

 

No, but U2C does.