Why was that Sephiroth fellow there?

[The Crystal]

In answer to the original question of the thread, "why was Sephiroth there"... AC is about wrapping up Cloud's story - the two main threads being the lingering guilt he feels about the harm he caused, which is primarily centered around Sephiroth and Aerith. It's about him being forced to confront and overcome his past so he can move on.

And AC is a great movie. The problem is that if you want to understand the story, you have to... Play FFVII, read all the stories of "On a Way to a Smile", and read the Reunion Files. lol