The word is used for people you don’t know the name of, or that you’re very close too.
In Japanese, as soon as you know a person’s name, you’re expected to use that, even when talking directly to them.
Japanese people on language exchange apps don’t mind using because they expect people to be not good enough in Japanese yet to make that distinction.
Just know that normally only good friends (or even lovers) use . And that you probably shouldn’t use it anymore as soon as you know the person’s (sur)name.
I’ll give examples from my own experience in Japan.
> Every time I met a new person, they’d call me (or an equivalent to it). But as soon as they knew my name they stopped using it. After I became close friends to a couple of people, I got called again by those people, or just my name without a suffix.