You need a certificate if people leave personal data on your site: they pay with a Bankcard, create accounts, and subscribe to mailing lists.

It is not safe to enter this information on a site without a certificate — it may be intercepted when it is sent to the server. If you install a certificate, the browser encrypts the data before sending it to the server. Even if fraudsters intercept the information, they will not be able to use it — instead of the card number, there will be a set of random characters.

The choice of an SSL certificate depends on the internal structure of the site: whether the site has pages on subdomains or whether the site is accessible from multiple domains.