Because every few years (4, I think), the number of bitcoins in each "block" (what you generate) is halved. So right now you get 50 BTC when you generate a block. In a few years it'll be 25. Then 12.5. Etc. In a very long time, they'll round from 10^-8 down to 0, but generating a block will still get you transaction fees from the transactions that occurred since the last block generated.
I asked why, not how :) I understood from the FAQ how it was done, but there's no mention of why it's desirable? Why not either fix the number of bitcoins now for all time, or let them grow unboundedly at a constant rate?
Ah, ok. I think they're trying to balance 2 interests:
They don't want inflation. One of the impeti for this initiative is to have a currency that some politicians can't decide to print more of.
But at the same time, they can't just issue 90% of the total money that will exist right at the beginning. No one's going to buy into a currency where the vast majority of the "wealth" goes to a few early adopters.
So it makes sense to have the money supply grow fast at the beginning, when the currency is hopefully getting more popular as well, preventing inflation, and then to gradually slow down the money supply growth to achieve the goal of preventing inflation.
