SSL stands for Secure Sockets Layer, SSL creates an encrypted link between a web server and a browser. It prevents customer information from being stolen. Credit card or banking numbers, name, address and other personal information will remain secure.
Whenever you need to gather personal (Contact form, login form), sensitive information an SSL certificate is a must.
Most hosting providers are giving FREE SSL (self signing) now so no more excuses of running a website that is not secured. Your SEO will also be affected if you run without SSL.