Gold, What is it good for?

There is one tactical benefit to gold.
If you have a large quantity in a city, and an enemy hits with troops (enough to breach walls), the proportion of gold to over-limit resources matches the proportion taken by the enemy. If the attacker sends just wagons, the gold has no effect since you need troops to breach the walls.

Example
On hand/over limit:  100m gold, 2m food, 2m wood, 2m stone, 2m ore

Attacker sends 890 wagons and troops to breach walls

They will get 6.8m gold, 255k each f s w o

Attacker sends 890 wagons with no troops

They will get 2m each f s w o

More gold means you lose less over-limit res from a troop hit.

Attacker Note
If you attack with troops (to break any small walls) and you see a line like this:

Gold 2m  food 20k  wood 25k  stone 18k  ore 19k

You should send another hit with just wagons since they might have a hoard of gold protecting their res.