Hi,
several reasons:
* to suppress common mode HF noise between communication partners.
* to avoid high voltage (static charge) to build up between communicaton partners.
But:
You should avoid to (possibly) create GND loops.
If you hard connect both GNDs:
* there is the possiblility for HF and LF down to DC GND loops
If you use a (series) capacitor:
* you avoid DC and LF GND loop, but still HF loops are possible.
* there is no suppression of static charge.
Thus use a capacitor in series with a low ohmic resistor.
* the resistor value needs to be high enough to lower the Q of a resonance, but low enough to still suppress common mode noise. (maybe 10R ... 100R)
* still there is no suppression of static charge.
Add a high ohmic resistor in parallel to the RC combination.
* This resistor in the MegaOhms just should suppress static charge.
(This is what I usually do)
But in detail it depends on both devices and the environment (noise, static charge...), maybe more..
Klaus