imho -
These were (/are) historically terms of endearment. Most meaning 'the other half'.
Or, if you think more deeply, the other part of me (the 'completion' of me).
Usually, they were used by very close couples and hard for others to really understand (often, because there were underlying details only that couple knew).
There are (obviously) both male and female equivalent versions of these names.
However, they fall in and out of favour between generations. Sometimes, resurrected by culture of the time (a line from a song, perhaps).
Some, usurp these names for other reasons (including political and social) for their own agendas.
But, those that used them (within their own lives) mostly did it with only the best intentions. Intentions, generally lost on those looking into the relationship (and that was, somewhat, the point).
Peace.