`--run-dir` was used to allow you to point at another user's suite so I could use `cylc suite-state foo --run-dir=~hjoliver/cylc-run`. Similarly, it gives the capacity to point at a mirrored suite database which may not be under a normally accessible account/directory structure e.g. you might want to look at operational suite state but not have access to the account, only wherever the mirrored copy is put.