If you don't apply an SSL certificate to your website, there are lots of things that can, and will happen, some more noticeable than others.
The first, and most glaring issue is that your customers web browser will show the site as 'Not Secure', and quite often faces them with a huge red warning, or at the very least, a symbol indicating that the site is not to be trusted.
With SSL, an encrypted connection is made between your customer, and your website. This indicates to the customer that the website is secure, and all data that they enter into the site is secure too.
Contrary to popular belief, you do need an SSL certificate even if you aren't capturing personal data, or anything under GDPR.
Finally, if all the above hasn't convinced you of the importance, SSL impacts your SEO, and forms part of Google's website ranking criteria. Simply put, if you don't have an SSL certificate, you won't appear right at the top of search engines when people look for you.
"The Best Place to Hide a Dead Body is the Second Page of Google"