I love the expression “happens to be.”
We say it when we are pointing out something about someone’s social identity. The thing we are identifying is held, by some groups, to be a pejorative. Using the expression says that you are not one of those small minded people. You are mentioning a distinction in passing, but it is not that important to you. It doesn’t define how you see that person. You hardly even notice it really.
Except you pretty much only use the expression in a context in which the distinction actually is important. You would probably not say, for example, “I handed my friend Julie, who happens to be a lesbian, the book.” That would be weird.
You’re much more apt to use it when you’re speaking in a context in which the information about the person’s race/religion/political affiliation/gender identity is relevant. For example, you are talking about how the state of Virginia combined Martin Luther King Jr day with a celebration of Confederate soldiers and ended up with a compromise that pleases no one— “Lee Jackson King Day.”
One of your friends had something pithy to say about this and, by the way, she “happens to be” African-American. In this context, you bring her race up because her perspective as a person of color is actually a relevant part of the story.
Yet you don’t want the listener to think you just go around all the time calling Lois “My Black Friend.” “Happens to be” signifies that we’re comfortable with the difference we’re pointing out. That’s what we’re trying to say with the words. What we’re also saying, less intentionally, is that we’re uncomfortable talking about this difference. That’s a lot of work for three little words to do.