App devs can build any app they want on top of these web3 protocols, but their apps may have to pay a fee to interact with those protocols. The fee incentivizes

App devs can build any app they want on top of these web3 protocols, but their apps may have to pay a fee to interact with those protocols. The fee incentivizes the actors that host (run nodes for) the protocol. That fee may or may not be passed on to users. It's a bit like paying a microtransaction for certain HTTP requests. The amount of times that a user has to pay a fee can be minimized through various pieces of technology like rollups (read about zk rollups and optimistic rollups if you're curious).