Why do some parents adopt a child after they already had biological children?
[Edit: thank you everyone for your responses! You helped me realize things I hadn't considered and opened my eyes to some biases I had. My understanding of adoption was very simple before I asked this question and the answers helped me understand it's much more complicated.
I remember in middle school there was a boy in my class who was the oldest of four children. Him, his younger sister, and the second youngest brother were all the biological children of their parents. However the youngest brother of that family was adopted.
I know the most common reasons parents adopt is because of infertility or because of being a same sex-couple (myself, I always thought if I ever become a mother I would adopt because I have a phobia of pregnancy and I am not bothered of the idea of having a child that doesn't look like me). So I was and still am curious as to why a family with already a lot of biological children would then adopt another one. I never ask that family why because I didn't know them very well and I figure it would be a kind of rude thing to ask, but the question has stuck with me for a while.
I'm not sure if this is relevant context or not, but I remember the parents of that family being an interracial couple (East Asian and white) with the children being white-passing biracial. The toddler they adopted was visibly full East Asian. Maybe that's weird to say but I figured I'd mentioned it because I know interracial adoption is a source of ethical debate so I wonder if race has anything to do with the decision making.
Anyway, I was wondering if anyone had some info or theories that could satiate my curiosity, or if anyone had personal experience being a parent or child in a similar family situation.