a way to look at it is to change gc and credit to USD and Euro for a moment. on one continent (i simplify the canadians away
) you pay in USD and on the other continent (also simplified) you use euro's.
on both continents one currency is dominant, because the other is 'not working' there. but when you go shopping abroad than you need that other currency. but why would they sell that? only when they want to do some shopping your side of the Atlantic.
The rate between Euro and USD represents the desire/need to change currency of both sides.
another example is the dual currency diplomacy of certain third world countries. their own currency is too unstable to rely on. therefore all imported goods and most 'very expensive' goodsare traded in USD. the usd is very stable in regard to the nations currency. so is the credit a more stable currency than the gc. voodoo is imported, turns are exported and ships are too expensive to use the unreliable gc for.
a fixed low rate exchange service reminds me of the socialist system of government regulation. they set rates that in no way represent the real value. in one SE-Asian country was the official rate 1 usd to 6 gc. on the market it was 1 usd to 1325 gc. a coke sold for a 900 gc. that was officially $150 but in reality less than $ 1