SSL certificates keep online interactions private even though they travel across the public Internet, and they help customers gain the confidence to transact with your web site. 

If you ask users of your web site to sign in, if they enter personal data such as credit card numbers online, or if they view confidential information such as health benefits or financial accounts, you need to keep the data private. You also need to help them confirm that your web site is authentic.

SSL is also used for email servers, web-based applications, server-to-server communications and more.