SSL is used when you want to protect the data sent between, for example. a browser (client) and a server or between different servers and applications. Perhaps the most common example is credit card payments, then you obviously do not want to send credit card information in clear text. SSL should be used when you value safety and expect others to trust the service providing
An SSL Certificate also tells what address it is issued for, and in some cases, depending on the certificate type, which company is behind the website you visit. This information is displayed by clicking on the padlock in the browser or on the seal that comes with some certificates. Is the certificate of type EV (Extended Validation), the address bar turns green and who issued the certificate, and who it is issued for, are displayed to the right of the address bar.