This tab would sort items by the amount of users watching a specific item from high to low.
To watch an item in the case of my usage also means to favor it for later reading or because you would like to set up a list of items you really would like to save.
With this in mind users would like to know which items other users are watching to expand their own list.
I think it would be a nice addition since users can watch items at all.
If you don't think this is a good usage of the watching feature then a special designed favor feature may is something to consider.
It would be useful for my own community. We use showcase to display information about voicebanks for a singing synthesis software. We have encouraged our community to use the "watch" and "update" features to publicize updates and receive notifications of a new version's download link. Since I disabled rating due to certain people trolling, seeing how watched an item is is a great way to gauge real interest in an item beyond just its views.