When to seek out a private investigator

In the movies you see people hiring a private investigator to look into a cheating spouse or help with finding a long lost relative. However, the other day I saw a billboard for an Arizona investigator and couldn’t help but wonder why someone might actually hire an investigator. I sat in my car, at the red light, just pondering the thought. What do they do, how much do they cost, why would I need one. These were a few of the many questions I had. Before I knew it I was daydreaming about the various circumstances that might require me to hire my own investigator.