It depends? As always, it depends on your specific community. As you said, it already notifies you if someone replies to you so mentioning someone is just a way to grab one’s attention so they really read your post.
Sometimes I mention people when replying because they have a strange username AKA not a name. (example).
In my opinion it sounds odd to do this: “Hello ExyTa165A” in comparison to “Hello Stephen”. So I just @ them if they don’t have a name as a username.