I intended to make transaction fees optional. Currently, many transactions can be sent for free, but a small fee helps ensure that your transaction is processed promptly when the network is busy.
As blocks fill up, fees will rise due to market competition. Users who need fast confirmation will include a fee, while those who can wait may send with no fee.
Transaction fees serve two purposes: they compensate nodes for processing transactions, and they make it expensive to flood the network with transactions.