Is Sona good? What's her role?

I'm a support main and I got her to pad out my support line up. I don't regret buying her, but I feel like she's sort of underwhelming. I looked up an item build for her, but even with that I don't feel like she's better than Nami in pretty much any significant way. On top of that, I feel like it takes more items and gold for her to be useful than anyone else does, and I feel like her abilities cost a lot and don't do that much, even when we get out of laning phase and her abilities start affecting more people.

What is her role exactly? Does she counter certain matchups that Nami wouldn't? I don't think she's terrible, but I'm almost to mastery 4 with her and I'm sort of having trouble finding reasons to pick her over other enchanters. If I want healing Soraka is better. If I want CC Nami is better. If I want to be more of a bully while also protecting the ADC then Morgana or Lux are better. If I want shielding then Janna is way better.