I've sometimes wondered this; in my usual habit of maintaining a large treasury (before Wall St, which as vmxa points out does make your money work for you.... obviously CivIII came out before recent events

) I stop and think - why not just rush for that next tech at -35 a turn? What is this money for?
Two things you don't mention are
a) Espionage. From the initial setting up of embassies and planting of spies, to city subversion, sabotaging production or seeing where enemy units are.
b) Trade includes military alliances; either to set one up (if you're dominating) or to bribe one or more enemies into making peace (if you're being dogpiled).
I don't know if your treasury enters into your "powergraph" rating, or whether that determines how aggressive enemies might be towards you.
Incidentally, a great way to get rich in the early game is barb-bashing. Once you've got Horses/Horseback Riding, it's a walk in the park to wander about sacking villages for 25gold a time. The downside is that as you get rich, everyone wants a piece of you...