There are various validation processes utilised, for the different types of SSL Certificates. One of the main ones being a real-time automated email verification.

Domain Validated (DV) SSL Certificates always use an automated Approver Email system.

The verifications are done to ensure that the domain owner is in fact the holder of the SSL Certificate. At the end of the day an SSL Certificate is used to satisfy your website users that you are who you claim to be.