Bitcoin and Liquid Fees
Fees are paid so that your transaction will be included on the blockchain.
Note: Bitcoin fees are collected by bitcoin miners, not Blockstream.
Bitcoin and Liquid 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.
Note: Green quotes fee estimates, which don't grant a specific confirmation time, just an estimate of it.
Fees in fact vary over time depending on the current traffic on the network, which might spike suddenly from time to time. In these cases the fee estimates become unreliable all of a sudden, and your transaction might be stuck for a while. In these cases you can always increase the fee later on if you have urgency to get your transaction confirmed.
Tip: You can optimize on your fees spending. Simply select custom fees and enter a feerate expressed in sat/vb, you can find optimal values on websites like mempool.space or whatthefee.io.
Lightning Network Fees
Transactions on the Lightning Network are facilitated by nodes who let your transaction flow through their own channels. Their open channels have some bitcoin liquidity locked inside, therefore some nodes (especially those well connected and with large liquidity availability) ask for a small fee for their service.