Private Contacts is an Android app to manage your contacts with additional features for privacy and data protection not offered by your phone’s default contacts app.
When any app (e.g. a messenger like WhatsApp) asks for permission to access your phone’s contacts, the choice is purely binary: You either give it access to all your contacts or none of them. In that moment you will likely feel fine about sharing the phone-numbers of your friends who also have the same app installed, anyway. However, what about the phone number of your doctor, your therapist, etc.? Do you really want to provide that information to everyone asking? Often, the mere presence of that contact on your phone is enough to deduce a lot about your health, preferences and behavior.