If the website doesn’t have a security certificate, the data is very easy to intercept or modify. The HTTPS protocol is the world's most up-to-date standard for securing data in the network, which so far hasn’t been broken by anyone. The operation of the SSL certificate is based on an algorithm that encodes data with a 256-bit AES key. So, even if a third party manages to intercept your website traffic, customer data will be safe.
Please note that Google favors sites with an SSL certificate, while also sending an alert if the site does not have one. The red Not Secure message will scare customers away, which is why it’s so important that your store is secured.