Whats the point of playing Ahri?

Don't get me wrong - I like playing Ahri but... what's the point in playing Ahri if I can play Lux (Just as an example)? Ahri really doesn't seem to contribute to the team as a whole like some other champions. (Again using Lux as an example) Lux can cast a shield which is useful to the team. Lux has a stun which is useful to the team. Lux can slow, which is useful to the team. But Ahri (and assassins) only effects single targets with the exception of her W and Q, which doesn't do enough to make me feel like I'm actually being useful to the team.

Uh, don't hate me guys - I just really, really want to have a 'reason' to play Ahri. I really want to play her, but I feel like I just... contribute more, when playing Lux due to the nature of her abilities.

If anyone can prove me wrong, please do.