An SSL certificate encrypts sent a recieved data making it very hard for packet sniffers to see your user's passwords and details they put through your site as well as what you send them. Depending on the SSL you buy depends on the added benefits that will help build trust with your site visistors or customers, so they know they are safe.
A lot of web broswers will flag your site as unsafe and warn users against visiting if you haven't got a valid SSL. This can drive a lot of people away.
You can read our article on why you need SSL by clicking here