First Time Dreamer
Ariadne is us and we are her. Just like the audience, she is thrown into this crazy world of dream sharing that she had no idea existed. She is a little confused and scared but mostly intrigued by the endless possibilities of authoring her own dream world: "pure creation" as she calls it.
She's our proxy. If there weren't a new person in the mix of the otherwise veteran team, they would have no reason to explain anything. Luckily for us, Ariadne needs to be taught what is what in the world of dreams and so we can learn at the same time. Unfortunately, unlike Ariadne, our dreams don't explode in slow motion. That is so cool. We want.
Ariadne isn't just a convenience for the audience, though; she also has her place in the film as the innocent novice who is, as Miles says, being corrupted by Cobb and his posse of illegal dream hackers.
In the context of the film the whole dream invasion thing seems pretty innocuous. They're trying to stop the monopolization of energy by a powerful corporation: doesn't that sound like a nice, philanthropic thing to do?
However, if we think of extracting or incepting in a larger context, it can get pretty sketchy… especially if we pay attention to Cobb when he talks about how much an idea can change someone and how it can grow to define them. Ariadne's innocence, however, never gets corrupted by any of this. She stays pure of heart.
Ariadne the Healer
While she may be innocent when it comes to dreaming, Ariadne is no slouch at relationship counseling. Originally, she was never meant to come into the dreams; she was only going to build them. However, after experiencing what Cobb is dealing with and the dangers that Mal could bring to the expedition, she decides she needs to come with the team because Cobb needs someone there who knows what he's dealing with.
When Ariadne is suggested to be the one to guide Fischer to the safe room, Cobb quickly says, "No, she's with me." It is with Ariadne's urging that Cobb is eventually able to shoot Mal and it is Ariadne who's with Cobb at the end.
It is only with her help that Cobb is able to finally let go of Mal and the (often very literal) pain she's been causing him. Before entering the dream she tells him that he needs to confront Mal, but that he doesn't have to do it alone.
She hears the truth of his story, literally kills his memory of Mal, and then reminds him not to lose himself again in Limbo. Her innocence does not stand in her way of healing the hole Mal left in Cobb's heart.