why is a GROUND electrode needed with a referential amplifier?
Differential amplifiers such as instrumentation amplifiers, with or without unity gain buffers, have a certain working range for the common-mode voltage. 



In general they will stop working if either input voltage gets too close to the power supply rails. For example, in the case of the INA117, both input voltages must be within the range of V+-4V to V-+4V, so if you have +/-10V supplies, each input must be within the range of +/-6V. There are also internal nodes in instrumentation amplifiers which can saturate. Here are some curves for the INA118 which show the permissible input voltages: 



The reference ground makes sure that the inputs remain within the common mode range for proper operation and minimizes the common mode AC voltage. The latter will show up in the output even if the inputs remain within the common mode range, attenuated by the common mode rejection ratio (CMRR).