For the hypnotist, it can be a gauge of depth of trance. It indicates to the hypnotist that his subject is accepting the suggestions that are being given. Depending on the type of suggestion and the level of compliance of the subject, there would be an opportunity to analyze or troubleshoot the induction on the fly.

For the hypnotee, hypnotic phenomena ratifies the trance. Carrying out the suggestion of the hypnotist can act as proof that hypnosis is actually taking place, on some level. The experienced hypnotist will build on this ratification to continue to deepen and test trance depth until they feel that such demonstrations can stop and the hypnotist can move onto the next phase of the purpose of that session.

If the context of the induction is stage hypnosis (performance, entertainment) or a class/workshop setting, then the additional purpose would be for the audience or classmates to witness actual hypnotic phenomena.