Some users seem to sometimes just check primary for every alias they add, even overriding an alias they just set as primary seconds earlier - it's unclear whether they have any idea what the setting even does.

It would probably make sense to link to https://musicbrainz.org/doc/Style/Aliases#Localised_names from the "This is the primary alias for this locale" checkbox label, so that there's an easy way to find out how it should be used.