From the mid-game on, there is really nothing to spend your money on. Or rather, you begin to accumulate enough money that your costs fall far behind your income. Other than the ability to purchase fiefs, there isn't an obvious and immediate way to convert wealth into other forms of power (influence, troops, better companions, etc.) so it winds up becoming an accessory and so taking time to develop an economy is generally wasteful.
Instead you can get any and all of those benefits instead from beating up bandits, mercenaries and lords.