Do we need to connect earth to instruments working at 24 volts? If not why we are earthing the instrument JB?

The24V JB are earthed only because the material of JB is metallic due to which it may come in contact with some electrical power circuit through cable tray or so and current may flow through it. Hence all the JB shall be provided with earthing. Also if JB is not metallic then there is no point of earthing the Junction Box. 

Instruments can be earthed but care has to be taken to isolate them from body earth. The Panel earth and Instrument earth should not come in contact inorder to prevent damage to instrument.

Earthing an instrument is security for any ambeint/unwanted interference. Earthing the JB is helpful to earthing all the instruments. Earthing is necesary for smooth automation.