If you have SSL the address for your website changes from http:// to https:// (the s stands for secure) and all data is encrypted. This is especially important if a website collects private data like e.g. credit card information.
With an SSL certificate it is clearly shown in the browser, that the website is secure. Often you can tell by a small green lock or a text saying "this website is secure". This makes customers feel safe.
Google and other search engines ranks websites with SSL higher in the search results. Therefore, with SSL you are able to make your website more secure and at the same time get a better placement in the search results.