Food and/or drugs. When two animal species have an intimately codependent relationship like this it's called symbiosis, and it's a very interesting field. Some ants keep herds of aphids, a tiny insect whose secretions apparently taste like delicious caek.
For evolution, the answer tends to be "why not?"
And also very often: those who don't do it can't get to have successful sex.
I think the article touched on this when it said the crabs use the anemones like nets to catch food and ward of larger predators.
No need for questions: God works in mysterious ways.