Steve Rogers: You know, I really miss the days when the weirdest thing science ever created was me.
Ultron: [speaking to Quicksilver] You and I can hurt them, but you [turns to Scarlet Witch], you're going to tear them apart from the inside.
Ultron: I'm going to show you something beautiful...everyone screaming for mercy. You want to protect the world, but you don't want it to change. You're all puppets tangled in strings.
Chosen answer: The pool allowed Thor to relive the vision that Scarlett Witch made him see. It's through the vision that he finds out that Loki's staff contained an infinity stone and it would be to dangerous to destroy. He also sees the creation of Vision. He also sees that Asgard is in trouble. Since Selvig is an expert (as much as one can be) in Asgard and Earth he would be the one to help Thor find the pool. Many of the scenes relating to the pool were cut and could appear in the home release.