Fees are a function of transaction size and network volumes (how congested the network is). Transactions are processed in blocks, storing up to 1MB of data. Hence, a sudden spike in activity often leads to network congestion – transactions waiting to get verified. In such situations, miners target transactions with higher fees first. In other words, the more a user offers in fees, the faster their transaction is likely to be verified.
Source: https://www.coindesk.com/markets/2023/03/23/rising-bitcoin-miner-revenue-from-fees-suggests-onset-of-major-bull-run/?utm_medium=referral&utm_source=rss&utm_campaign=headlines