there are a lot of leverages for money i think, although difficulty has some affect on that i think.
i'm a horribad noble player only.
for all eras of the game, being able to keep your research slider at a resonalbe rate is very important to all phases of your development trajectory.
also,
early game:
- fund expansion (offset maintenance costs)
- keep a big enough military to appear resonably disouraging to your warmonger neighbors in the early years
mid game:
- again, power ranking
- funding taken-over cities, they can cost, especially if distance-from-seat-of-government is significant
- be able to buy with cash happiness and/or health resources if you have no resource to trade
industrial:
- upgrading of experienced units (can be the difference of losing and winning a war)
- close your borders to a rival to rob him of you as a trading partner (this can actually be pretty powerful if the trade routes between you and him are of high value)
- ability to run a tick of the culture and/or espionage slider in time of war and still keep research up
and, if you're really rich, you can bribe a moron like ragnar into fighting your enemies for you, just pure cash.
the more i play BTS, the more it seems to me that money - especially during the first two eras - is everything to my ability to compete later in the game.