What was the reason for dns validation?

I got a certificate from AWS and did dns validation in the process of https communication. But I'm not sure why you are doing that verification. To use the domain, I got a domain from a hosting company and registered it on route53. Isn't this process itself dns validation? I want to know the effect of dns validation, and I want to know what happens when dns validation is not done.

thank you.

From AWS Option 1: DNS validation:

When you choose DNS validation, ACM provides you with one or more CNAME records that must be added to this database. These records contain a unique key-value pair that serves as proof that you control the domain.

With shared infrastructure, the domains of multiple customers may point to the same IP address(es). Therefore, this alone cannot be held as proof of control for any specific customer. That is probably why these records has to be unique key-value pairs.