This explains it very good in a different way. This info is not enough for all boards.
I will use a screenshot from Meta.
If you imagine most recent private messages are send from one topic, so all titles are the same.
This image is not that bad. But when you are in groupmessages and have a lot similiar (topic-)titles it’s not easy to see whose PM you’re opening.
The white space below title could show all recipients comma seperated.