To be pedantic, they help secure transactional *behavior*. The network is still secured via consensus. The fees operate as a sort of dynamic consensus, like your own white blood cells, hunting down bad behavior and causing them to become very expensive.

But bad behavior, like an infection, still affects the network. And it can leave a permanent mark (scar) in the form of unspendable utxos and larger historical blocks (either as OP_RETURN, witness, or other data).

And like an infection, whatever doesn’t kill the host will make it stronger.