For a long time it is already possible to leave comments on files. With Nextcloud 11 we provide a way to mention users in comments, so that a notification is created for the respective individual. This was never really advertised, however, because it was lacking an auto-completion tool that offers you a type-ahead selection of users. The crucial point is that user IDs are neither known to the end user nor exposed to them. For instance, if users are provided by LDAP, it might look like "1d5566a5-87e6-4451-bd2f-e0e6ba5944d9". Nobody wants to see this and the average person also will not memorize it :)
It would be sad to see the functionality rot away being unseen in a dark corner. With Nextcloud 13 the time was ripe to finally include this missing bit, which is actually pretty fundamental and every application that allows text based communication amongst multiple people ships it.