Fees are paid so that your transaction will be included on the blockchain. Note: Fees are collected by bitcoin miners, not Blockstream. Transactions are considered confirmed when a miner (on Bitcoin) or a functionary (on Liquid) includes your transaction in a block. Blocks are added to the blockchain every ~10 minutes (on Bitcoin) and every ~1 minute (on Liquid), and the blockchain provides a history of all transactions that have ever occurred. There is only a certain amount of space in each block, which means not all transactions can be included in the next block that is added to the chain. Because of the competition for block space, users will pay a fee to incentivize the network to include their transaction on the blockchain, and to prevent the network from being bloated with spam transactions. The higher the fee, the more likely your transaction will be confirmed sooner.