As @colman has said here, I think that there are use cases for all variations that would benefit from having two fields.
For example:
one-to-many:
employee being subordinate of other employee - it is nice to see both ‘manager’ and ‘direct reports’ for a given entity
one-to-one:
rowing partner in a coxless pair - one person will be rowing strokeside and one will be rowing bowside, and it’s nice to show which is which
many-to-many:
team members, where one person is the team leader (+ a person may be leader of more than one team and/or may be (non-leader) member of more than one team) - being able to show leadership and membership roles separately might be useful.
To be honest, this last one probably lends itself to creating a new type (= team) with a many-to-one relationship to team leader and a many-to-many relationship to team members, or even some other solution.
Anyway, they were just the first examples off the top of my head.
Of course, it’s possible to think of examples for each type of relationship where only one field really needs to be shown, so I’m not saying that having two fields is always right. Perhaps when the fibery team release the ability to hide/show fields, then the problem goes away, since showing only one field is obviously a subset use case of showing both 