who, alot of complicated ideas flying about here, so i thought i would just simplify things;
a conventional earth is not a stick into the ground, consider it a 0v reference point with a very low impedance (IE lower impedance than through your body) therefore an attractive route to compete a circuit for any stray current (IE from a loose wire or whatever) if it were to touch a metal case, and somone was in contact with that case- should the case be earthed the current would go through the earth line (AC AND DC ALIKE) instead of through you.
In an AC it is for exactly the same reason, remembering that for a current to flow there has to be a potential difference (if you and the aircraft are all at 10,000v and you touch a 10,000v wire it doesn't matter because there is no difference - hence how birds sit on power lines). Therefore an ACs earth can merely be a mass connection of everything metal that stops a difference from forming between different parts - IE connection to the neutral of the battery too. A loose wire would therefore short through the low impedance mass of metal and not through the higher impedance human flesh. On the ground, the AC ground can be connected to the real world ground so there is no difference between the AC and the rest of the world - remembering it is a difference that causes a current. you do not want this difference to equalize through the fuel hose - a grounding reel is much more preferable.
source: electronic engineering degree