Not having an SSL certificate would lead you to miss a key signal of trust for your visitors, which will result in lower conversion rates and a lower return on investment. Google's algorithm penalizes these types of sites as a way to protect their users and will rank them lower on search engine results pages. This will trickle down and lead to fewer visitors. Visitors who don't know your site is not secure are at risk of getting their personal data stolen by the site's coding that is no longer up to date with the latest security standards.