Truecaller is one of those "it just works" apps. It is completely transparent to the user. You can use it as a dialer to make and receive calls.

A lot of people don't understand that it's silently uploading their contact lists, personal information and call behavior off to the internet.

The customer service agent in the article from truecaller is correct about the usefulness of the app. A friend of mine was able to track down the owner of a fraudulent dishwasher service center in Bangalore because of truecaller. The photo and name of the owner in the app attached to the business mobile number.