I don’t know if the tape that was on JonBenet’s mouth matches that which came with the doll or not
(attached below is a picture for reference). But as for
why it was placed on her, I’d have to guess it was to answer Patsy’s
rhetorical question that she asked
repeatedly after JonBenet’s body was “found”.
From Wikipedia:
A rhetorical question is a figure of speech in the form of a question that is asked in order to make a point. The question, a rhetorical device, is posed not to elicit a specific answer, but rather to encourage the listener to consider a message or viewpoint. Though classically stated as a proper question, such a device may be posed declaratively but implying a question, and therefore may not always require a question mark when written. Though a rhetorical question does not require a direct answer, in many cases it may be intended to start a discussion or at least draw an acknowledgement that the listener understands the intended message.
A common example is the question "Can't you do anything right?". This question, when posed, is intended not to ask about the listener's abilities, but rather to insinuate a lack of the listener's abilities.
Although sometimes amusing and even humorous, rhetorical questions are rarely meant for pure, comedic effect. A carefully crafted question can, if delivered well, persuade an audience to believe in the position(s) of the speaker.
In simple terms, it is a question asked more to produce an effect than to summon an answer.
Patsy’s rhetorical question was, “Why didn’t I hear my baby?” The intended effect of the question was to make anyone believe the tape was placed on JonBenet by an intruder in order to keep her quiet so no one in the house would hear her, and thus were unable to come and save her.