Because of the second bullet point of the Charmed condition.

Charmed
• A charmed creature can’t attack the charmer or target the charmer with harmful abilities or magical effects.
• The charmer has advantage on any ability check to interact socially with the creature.

If you're planning on making social checks at advantage a lot, they'll be more likely to succeed if you have a good Charisma. And can better top results, since advantage doesn't add to the maximum result possible on a check.