You've got a website for your business and most of your new customers are reaching you via your "Contact us" form page. You're probably manually adding your customers' phone number, email, their preferred product, etc. in your Google Contacts. Would it be nice if your "Contact us" form page has the ability to save a visitor's contact and other details to the database of your account's Google Contacts app automatically? With this kind of application, server to server authentication is needed.