Question

why should we use a newly created administrator user and not root account in aws

Answer 1

The root account has full access to all the resources in the account. You cannot use polices to explicitly restrict access to any kind of resources with this account. 

With IAM, you can securely control access to AWS services and resources for users in your AWS account. For example, if you require administrator-level permissions, you can create an IAM user, grant that user full access, and then use those credentials to interact with AWS. If you need to modify or revoke your permissions, you can delete or modify the policies that are associated with that IAM user.

Reference: https://docs.aws.amazon.com/general/latest/gr/root-vs-iam.html