SSL certificate adds the security to your all data over the internet. It will provide the data integrity. Also, it securesyour details like the credit card or other sensitive information. When you are using any payment gateway like PayPal then it's compulsory to have the SSL installed on your website.
Sites having SSL installed on it have the more traffic, compare to the non-SSL sites.When end users see the green padlock while accessing your website on top left corner of the web-browser then it indicated him that it's trusted site. If there is no SSL then any computer connected to your server can see the username, password and other credentials on your site.When there is SSL installed on your website that time data on your website will be sent in encrypted format on internet so no can hack it.
So it's always recommended to have SSL on you domain.
SSL become main aspect in SEO ranking because clients used to trust and access the sites which are SSL protected. If your website is eCommerce site then customer used to do payments only with SSL sites. SSL sites load faster than non-SSL site so it's automatically increase the page ranking.