If you're thinking that you don't need a reverse phone lookup, let me give you a few examples of why you do need it.
Your Spouse
If you've noticed your spouse getting calls from people you don't know, this service is for you. It may indeed be totally innocent, but knowing for sure is worth the small fee they charge to find out.
Your Employees
If you have employees that use company phones, a reverse phone lookup is essential. Chances are, you won't know who they're calling most of the time. Better to know sooner rather than later if they're talking to your competition, or using your time and phone for personal business.
Your Kids
Every parent knows how important it is to know who your kids are talking to. Certainly all parents are aware of how many predators there are, and how vulnerable kids can be. Try as you might, it's hard to keep tabs on them all the time, so having a service like a reverse phone lookup is a lifesaver.