Why do I have to pay transaction fees on Bitcoin transfers? Why do I get the message "This transaction is over the size limit. You can still send it for a fee of 0.0005 BTC"?
For transactions which draw coins from many bitcoin addresses and therefore have a large data size, a small transaction fee is usually expected. The transaction fee is an incentive on the part of the bitcoin user to make sure that a particular transaction will get included into the next block which is generated. See also https://en.bitcoin.it/wiki/Transaction_fees