The blockchain concept is used kind of as a distributed timestamp system.
Say you send out the same coins to two different people at once, in a centralised system, that would be detected at the centralised server and the second transaction would fail. With the distributed system that's not possible, so the block chain serves as a timestamp. Whichever transaction gets into the blockchain first is the successful transaction.
However, generating the blockchain requires a lot of processing power, and therefore needs to be subsidized. Initially that is done by rewarding those who generate blocks with coins (i.e. mining). However, as the number of generated coins falls, transaction fees should make up the short-fall.
Say you send out the same coins to two different people at once, in a centralised system, that would be detected at the centralised server and the second transaction would fail. With the distributed system that's not possible, so the block chain serves as a timestamp. Whichever transaction gets into the blockchain first is the successful transaction.
However, generating the blockchain requires a lot of processing power, and therefore needs to be subsidized. Initially that is done by rewarding those who generate blocks with coins (i.e. mining). However, as the number of generated coins falls, transaction fees should make up the short-fall.