On average I guess alumni contribute more through donation than they receive through the financial aids. I don't know the numbers on this topic, so please pardon my ignorance.
Alumni of the university contribute in more than one way. But most importantly, they are considered an important network factor in getting the student adjusted and proud of where the student goes. Universities market their name as a good place to study. And children of alumni are the prime markets for them to get prospective students from.