- It eliminates one way of forging results. Without stating the seed, people could consistently fail to reproduce your results and you could always have "oops, guess I had a bad seed" as an excuse. You still have to worry about people re-running the simulation till the results look good, but that's handled via other mechanisms.

- Many algorithms are vastly easier to implement stochastically than deterministically. If you want to replay the system (e.g., to locally debug some production issue), you need those "stochastic" behaviors to nonetheless be deterministic.

- If you're a little careful with how you implement deterministic randomness, you can start to ask counterfactual questions -- how would the system have behaved had I made this change -- and actually compare apples to apples when examining an experimental run.

Even in your counterexample, the random seeds being reproducible and published is still important. With the seed and source published, now anyone can cheaply verify the issue with the simulation, you can debug it, you can investigate the proportion of "bad" seeds and suss out the error bounds of the simulation, etc.