Because conventional wisdom holds that polygraphs (i.e., lie-detector tests) are not generally allowed into evidence in criminal trials in this country (owing to their relative unreliability), many clients find it strange when their own attorneys suggest that they take a polygraph for a private polygrapher. What's more difficult for them to understand is the requirement that they pay for the "pleasure" of doing so. Because I happen to be one of those criminal defense attorneys who requires it in nearly every serious felony case I handle, obviously I have a different view on the utility of these private polygraphs.
Essentially, I have two reasons why I require most of my clients to submit to a private polygraph (and NOTE that I have not said that my clients would generally submit to the prosecutors' polygraphs--generally, they don't). 1) If my clients are able to pass a private polygraph, we are usually in a better negotiating posture with the prosecutor--and may get a dismissal or, at least a downgrade; and 2) Even if my clients don't pass the polygraph, that gives me a better understanding for the truth or falsity of what my client is telling me and it enables me to give them the counsel that actually speaks to what's best for them under the circumstances as they are, not as my client now wishes they were.
Navigating through lies and half-truths, trying to get to the bottom and give good advice is the hardest part of this job. Polygraphs, passed or failed, make that job many times easier.