Why can a manuscript copied from another surviving manuscript be less useful to an editor?

In the field of textual criticism, a manuscript copied from another surviving manuscript is often considered less useful to an editor because it is redundant for the purpose of reconstructing the original text.1

When using the genealogical or stemmatic method, critics aim to establish a family tree of witnesses to identify the "archetype," or the earliest inferable textual state. If a manuscript (the copy) is known to have been derived directly from another surviving manuscript (the model or exemplar), the copyist essentially repeats the testimony of the model. Consequently, the copy provides no independent evidence; its readings are only valuable if the original model has suffered damage or loss of text since the copy was made.1

By identifying such relationships, critics can discard these dependent witnesses, as they do not offer new information that would help distinguish the original readings from subsequent errors or variations.1