The goal isn't always to influence the judge.
What about influencing your doctor against you? Keep in mind that your employer and/or insurance company has already hand-picked your doctor. Many times we find doctors with a slant that favors employers and thinks that you might not be 100% truthful about your injury. So it might not take a lot to influence the doctor against you. In fact, we heard one reliable source tell on of our Attorneys that a frequently used doctor (now retired, thankfully) would sometimes tell the rehab nurse to privately tell the adjuster to hire a private investigator when he had doubts about a patient.
They can legally film you.
Yes, you can be filmed, but not through the windows of your house, PI's are not supposed to go onto private property. They sometimes do anyway, as soon as you have the lights on. Just keep an eye out.
If you have an attorney, the private investigator cannot ethically talk to you.
I've seen cases where they telephoned injured workers or bumped into them on the street. More than one person has come to me feeling threatened by a private investigator. Private Investigators are usually ex-cops, are in good shape and know the right tactics to intimidate someone or get favorable information out of them.
A picture is worth a thousand words.
But a video can really tell a story, and an edited video can take an innocent act and turn it into a night mare for a client and his attorney. Many times an insurance company will reduce hours and hours worth of surveillance video into about ten minutes of physical activity. They don't bother telling the doctor or judge the film was taken over a period of days or even weeks. We face these tactics all the time.