Questions about the gameplay of Heroes 6.

The meaning is the same; you will play as Anastasia, but as a different character. As for how to start playing, I wrote about it a little earlier: simply press + at the beginning of the campaign and create a new hero. As I wrote before, all game saves in this campaign will be lost. So be careful; you can try creating your character in any other campaign that hasn't been started yet.
And yes, regarding the meaning, it seems to me that it would have been simpler for the developers to allow choosing a special ability for the main hero instead of creating an unnecessary function.