When an incorporated line is not broadly recognized, the audience may mistake it for the new poet's own independent language. A verbal cue can direct the reader to the presence of an earlier poem.
For example, a new image may lead into an old hemistich; without a signal, the point of incorporation might be missed or misconstrued. Such a cue need not always be a full bibliographic attribution. It should be enough to distinguish borrowing from fresh authorship in the intended literary setting.
The essential aim is to prevent confusion between incorporation and independent invention.