Every cryptocurrency transaction must be added to the blockchain, the official public ledger of all completed transactions, in order to be considered a successful and valid transfer. The work of validating transactions and adding them to the blockchain is done by miners, which are powerful computers that make up a portion of the network and confirm its transactions. Miners spend vast amounts of computing power and energy doing this for a financial reward: with every block (a collection of transactions) added to the blockchain comes a bounty called a block reward, as well as all fees sent with the transactions that were confirmed and included in the block.
For this reason, miners have a financial incentive to prioritize the validation of transactions that include a higher fee. For someone looking to send funds and get a quick confirmation, the appropriate fee to include depends on the cryptocurrency being sent and can vary greatly, depending on a number of factors, such as transaction size and network conditions.
Your Blockchain wallet will automatically calculate the appropriate fee for sending your chosen cryptocurrency. The fee for sending ether is static (you can view the fee for sending ether by clicking “Send” and selecting Ether as the currency), while the fees for sending bitcoin and bitcoin cash are dynamic and will be calculated by your wallet after you input the amount you want to send. This article explains bitcoin transaction fees in greater detail.