An SSL certificate ensures all information transmitted to and from your website is protected from third parties attempting to access it. It is important because browsers, like Google Chrome, mark pages without https and the green padlock in the address bar as not secure. This can have a negative effect on the trustworthiness of your website. Pages that are protected with SSL receive better search engine rankings, and are more likely to be trusted by visitors.
WordPress is one of the most popular content management systems (CMS) in the world, and as such, many web hosting providers offer options that cater to this rather large niche. You can certainly host your WordPress site on a standard shared hosting plan, but by choosing a WordPress-specific option, you'll spend less time configuring and managing your WordPress installation.