Reference Material:

https://docs.aws.amazon.com/kms/latest/developerguide/key-policy-default.html#key-policy-default-allow-root-enable-iam
The following default key policy statement is critical.
It gives the AWS account that owns the KMS key full access to the KMS key.
Unlike other AWS resource policies, an AWS KMS key policy does not automatically give permission to the account or any of its identities. To give permission to account administrators, the key policy must include an explicit statement that provides this permission, like this one.
It allows the account to use IAM policies to allow access to the KMS key, in addition to the key policy.
Without this permission, IAM policies that allow access to the key are ineffective, although IAM policies that deny access to the key are still effective.
It reduces the risk of the key becoming unmanageable by giving access control permission to the account administrators, including the account root user, which cannot be deleted.
https://docs.aws.amazon.com/kms/latest/developerguide/key-policies.html