A SSL (Secure Socket Layer) Certificate is also referred to as a digital certificate. Digital certificates serve two main purposes.
The necessity to provide customers with a secure shopping experience has become very important. But even if you don't sell goods online, visitors may appreciate the fact that you are securing their private data, such as when submitting a contact form.
A very interesting excerpt from The Wall Street Journal:
Mr. Cutts (the liaison between Google's search team and website designers) has spoken in private conversations about Google's interest in [giving a boost in its search-engine results to websites that use encryption].
Paraphrased from Google’s blog, August 06, 2014
At Google I/O a few months ago, Google called for “HTTPS everywhere” on the web. In 2014 Google has been running tests taking into account whether sites use secure, encrypted connections as a signal in their search ranking algorithms. We've seen positive results, so Google is starting to use HTTPS as a ranking signal. Over time, Google may decide to strengthen https as a ranking signal, because they’d like to encourage all website owners to switch from HTTP to HTTPS to keep everyone safe on the web.