Currently, when using musicbrainz user to connect to PROD_STANDBY database, nothing actually prevents to accidentally enter edits such as https://musicbrainz.org/edit/88742537 (which was fortunately innocuous and is easily reverted with https://musicbrainz.org/edit/89602291) when the primary server and the second server are swapped.

Connecting as musicbrainz_ro would prevent making more changes (potentially harming) to the production database from a development environment.

It is mainly used for testing pages display and updating messages for translation.