Mining bitcoin is a speculative lottery. The bitcoins actually be transferred around were given originally to those who won that lottery, but after that, they're just traded like money. In the future, the winnings from the lottery is a percentage of each transaction instead of bitcoins, but that's relatively a long ways away. Basically, with each transaction, a person posts what percentage of the transaction goes to the winner, and the winner chooses which transactions to honor onto the blockchain. The giving out of synthetic bitcoins is a way to bootstrap this process and to make sure that there's no single point of ownership.