This is to prevent double spends
Double-spending is the result of successfully sending some BTC more than once. Bitcoin protects against double spending by verifying each transaction added to the block chain to ensure that the inputs for the transaction had not previously already been spent. These "confirmations" may take additional time to process.
When a Bitcoin transaction is made the authenticity has not yet been established. With Bitcoin's decentralized architecture, this consensus needs to form after your transaction gets included in a block and a chain of additional blocks extend from that. At any point in time there could be two or more solved blocks at the same level in the blockchain competing to win this consensus. At the time, there is no way to know which block will eventually attain consensus as being "the truth" and which blocks will lose support and become orphaned.