Normal website traffic transmit data in plain text. This can be dangerous when sensitive information such as credit card number, password is being transferred. All networks between the user and web server can see the information through clearly. When you are in public wifi, it will be even more easier for hacker to sniff it.
SSL encrypt data transmitted between user and web server. It encode the data to unreadable string. Thus the user information is safe from hacker. We can use same SSL to secure email and FTP.
SSL make customer more confident to shop with you. SSL shown as padlock icon on browser. With higher SSL, it bring green bar on the browser. Green bar describe that the domain owner has been validated, therefore trusted. More trust, more sales.
SSL require dedicated IP to work. One IP can only be installed with one SSL.