When you move a thread to another page, a placeholder always remains at the old page where it has been. However, unlike other threads on the page, it's not deletable. While it may be good to keep them around usually, it should at least be possible to delete them since they clutter the page and the table of contents and some users might not like them. Furthermore, on some wikis, it's common practice to delete unnecessary move leftovers. For example, on the German language Wikipedia, there's an own bot that puts speedy deletion templates on all talk page redirects and move artifacts with a parentheses title. Some time ago, a bot used to delete about 300 user talk subpage redirects leftovers from userspace drafts. Thus, a delete option would be useful, even though you might not want to encourage deleting them.