I relate to this question too. It's rather clever how it works, the forum currency has created its own economy with value because of the exchange rate to IGB. I created an account on The Bell Tree Forums a while ago, but I soon stopped being active when I stopped playing AC:NL. When I came back to forum when I started playing again, I noticed how TBT had become more valuable and users on the forum were really interested in obtaining it. I didn't get it at first, I had a lot of TBT in my wallet and I valued IGB a lot more (and still do) But I was like "Why is TBT so sought after, when everyone here plays the game? Surely IGB would be more valuable?" But then I realised that because everyone else wanted TBT, so did I. I think that's how it works. Because everyone wants TBT, exchanging TBT for services works in the same way exchanging IGB or even real money. Because there is a collective that shows it has value, it therefore, has value. It's a very clever system. In the same way money works in real life. Money itself is just paper and coins, but because everyone wants it and values it, you can exchange it for goods and services.
That's how I understand it, anyway. xD