Money is essentially a buffer you can use to redistribute production/farming etc, either by directly buying production (as in Moo2/Civ) or as grants to planets which boost all output bt a factor (as in Moo1). IMO its vital you have some way to do this in the game, some way to jump start new systems.
The other thing its very useful for is paying for empire-wide things, which cant easily be sourced from one particular planet, such as: fleet maintenance, spies etc.
So the only remaining question is : where does it come from? Either : Tax system outputs directly (as in Moo1), or make money generation a part of the system economy (e.g. banks, starports etc).
I don't know if we really need money; they could be just an unnessesary complication of the resource system. Trade would work fine without them, only with RP and PP.
Money is just a general abstraction of RP and PP (and food and minerals if you like). Its far
less complicated to have one generalised tradable/relocatable resource than to keep track of several independent ones. If you make players trade PP or RP (or any resource), then PP and RP become
money to all intents and purposes, but youve got different sorts of money which must be spent on different things.
Sure, you have the incongruity of being able to turn food into minerals (albeit to a limited extent, if youve designed the model correctly) but, to be blunt, so what?
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