The simple act of mentioning someone in a comment has more implications that you might have thought.
it adds the mentioned user as a collaborator. Meaning they get notified right away of the comment, and will be notified of any new activity on that task. Unless they remove themselves (and they often should to reduce the noise).
it makes it clear who you are talking to, especially when there are several collaborators involved, and if you have several times the same firstname.
mentioning someone that is already a collaborator is also a way to save a tiny bit of the energy required to check if that person was already a collaborator. Mention them anyway, worst case scenario they were already there, best case they weren’t and you brought them back!
Anything I missed?