Its used in combat.
You can then control them. There is also charm monster which does the same thing to monsters
Yes, charm person is also useful in combat. But it does give advantage to social dice rolls. Baldur's Gate 1&2 spells focused on combat, but in tabletop RPG's charm spells are great in social situations. There's nothing preventing this game from utilizing charms in dialogue. There's already a few cases where Detect Thoughts works in dialogue.
To make using those spells easier and more comfortable, they should be castable during dialogue.