They can expose an organization's dishonesty, or foolishness...

The (in)famous Sokal hoax served a valuable purpose in exposing the bias and lack of intellectual rigor in postmodern cultural studies...

George Jammal's Noah's Ark Hoax (Where he claimed a piece of a railroad tie he found near his job was actually from Noah's ark) served a valuable purpose in exposing the lengths some organizations would go to to promote obvious nonsense to advance their agenda. Discrediting and exposing those organizations.

In David Balsiger's most recent public statement about George Jammal's hoax, he writes that

There is something wrong with the ethics of the news media when they glorify the acts of humanist hoaxers who intentionally and successfully deceive 40 million TV viewers; and then blame the show producer and CBS for not discovering their elaborate hoax. This is not a case in which the producer or the network is guilty of deceiving viewers, but rather one more example of humanists who tout themselves as 'Ethical Humanists' being neither ethical nor honest when it comes to advancing their hidden agenda.[38]
Had circumstances been different, Balsiger would have had a point. If Jammal's hoax had really been "elaborate" and carefully constructed to resist anything less than the most scrupulous and detailed investigation; if it had not been filled with inconsistencies and intentional clues; if Balsiger had not been warned about Jammal being a hoaxer prior to the show's completion; if the show had not otherwise misrepresented and omitted facts; if Balsiger and Sun had a reputation for sober and accurate research, then his criticism would carry some weight.

There are, of course, serious moral questions which should be raised about the kind of hoax Jammal performed. Is the intent to discredit an entire worldview, or to reveal the inadequacies of particular organizations or individuals? If the latter, is the hoax the only way to bring public attention to these inadequacies, or are other methods available which would be about as effective? Are those being hoaxed given adequate chance to avoid falling into the trap?[39] Whatever Jammal's intent, his hoax has clearly demonstrated the inadequacy of the research of Sun International Pictures and brought it to public attention after letter writing campaigns and even books of criticism have failed to do so.[40] Sun had every chance to avoid being caught by the hoax, but disregarded the evidence and chose to produce a program filled with inaccuracies and misrepresentations. Now it must face the consequences.

James Randi used hoaxes to good effect to expose the 'serious reseach' of paranormal investigators as nonsense in Project Alpha, and destroyed the snake oil salesman Peter Popoff.

Hoaxes exposing the 'serious researchers' of cropcircles have exposed their lack of critical thinking skills.

The Rosenhan hoax prompted serious debate of psychiatric diagnosis.

The Rosenhan experiment was a famous experiment into the validity of psychiatric diagnosis conducted by David Rosenhan in 1972. It was published in the journal Science under the title "On being sane in insane places."[1]

Rosenhan's study consisted of two parts. The first involved the use of healthy associates or "pseudopatients," who briefly simulated auditory hallucinations in an attempt to gain admission to 12 different psychiatric hospitals in five different states in various locations in the United States. The second involved asking staff at a psychiatric hospital to detect non-existent "fake" patients. In the first case hospital staff failed to detect a single pseudopatient, in the second the staff falsely identified large numbers of genuine patients as impostors. The study is considered an important and influential criticism of psychiatric diagnosis.

In these Hoaxes a valuable purpose was served.... This BigFoot hoax helped destroy the reputation of some that advocate that garbage - people who were willing to claim obvious garbage was 'proof' in order to advocate their delusions. One of the hoaxers lost his job with the police department because it called into question his credibility- this seems justified as well, he chose to put himself at risk.

Other hoaxes have exposed the blind agenda of some who deny the reality of global warming.

This is not to say ALL hoaxes are 'good'- Piltdown man, and Archaeoraptor for example, did real harm to science.