The purpose of ISIS is exactly what they say it is. To establish their own state governed by hard-right Islamic ideals. And the reason they have so much support is largely driven from the perceived persecution of Muslims worldwide e.g. xenophobia in Western countries, drone strikes on innocent civilians etc.
And a ton of other news articles on the same subject. Pakistan is super dangerous because it is not all that hard to imagine a number of groups within Pakistan switching allegiance. It is also super dangerous because it is a nuclear power and has a long history of factions inside it trying to pull Pakistan into a much more radical direction.