Yes, to pay the miners to push thru your transaction.  I think it's not that the network is slow, but sometimes the transaction fee that you paid matters. Bitcoin blocks are almost always full. Since there is a capped size of 1mb per block, and each transaction is worth some bytes, there is a limited number of transactions that may be included in each block. Because of that, you need to "fight" with other users to get your transaction confirmed in the next blocks, this is done by using transaction fees.