Why a bitcoin confirmation is needed??

[zetaray]

A confirmation means the transaction is recorded on the blockchain. Anyone can send a transaction containing dust without a fee to pay for something and it may never confirm. Retailers wouldn't want those, so they need a confirmation. I think that is very reasonable.

[waser12]

The bitcoin's transaction is confirmed when it is integrated in a block and the recipient can not use it until this moment.

[mullerdan]

its like everything, you need a confirmation. When you buy something anywhere you have to receive a confirmation, it is safe for the buyer and for the seller.