I sometimes custom flag questions, asking a moderator to mark it status completed, because I believe the question served it's purpose, and most moderators mark my flag helpful, and actually add the requested tag.
But I'm not sure if that is a good thing to do.
In my experience, the tag is added mainly on MSE where people ask for new features to be added to the entire SE network, and this tag is added not just for feature requests, but for questions tagged discussion, bug, or support as well.
Currently the tag wiki reads (emphasis mine):
Indicates that a feature request has been implemented, a bug has been fixed, or another type of request has been processed.
What kind of questions deserve a status completed tag on ELU meta?
When should I request such a tag? Or should I never?