An SSL certificate is the most common protection against malicious attacks in which other people or machines could pretend to be you. You could call it an Identification Document, but for a domain instead of a person. It lets other internet users validate that all internet traffic coming from and to your domain is actually coming from you and not a hacker.
In the context of using Growbots, your experience will not be affected by not having a valid certificate, but in a very rare occasion of a so-called “Man In the Middle Attack” , a 3rd party might be able to read all the email data that you would be sending via Growbots. Having a valid SSL certificate negates that risk.
More often than not, your domain provider, like GoDaddy or Google, offer such certificates. In other cases, you can simply purchase a certificate online via any other provider of such certificates.