An SSL certificate is not necessary for the website to work (with our help section you already know, that hosting and a domain are required for the website to work).
However, if you want the website to be organically positioned well and have information for the user of your website, that it is a secure website - in these cases an SSL certificate will be necessary.
Its purchase and installation as well as correct configuration result in a clear message from the browser that the use of your website (e.g. by sending messages or making payments) is safe for the user, and his data is encrypted.
In short, it can be said that an SSL certificate is needed to maintain data security and to illustrate your professional approach to the maintained website.