There are lots of reasons. Not the least of which is that if your website doesn't have an SSL Certificate, you can expect to be in hot water with Google, Safari, and most other major search engine providers. Sites without SSL are automatically flagged letting people know they "aren't safe." And that's not a good look for you.
On the other hand, having an SSL Certificate is really good for SEO since Google and the rest prioritize secure websites. That means their search engines favor websites with the https protocol (SSL). In sorta plain English, you'll rank higher in the search results just by having the SSL certificate.