We will see why ?
Having only the Default-First-Site-Name, You would have not Define the subnet for your site,
If you have only one site , Which is the default website then All the subnet will look into your Default Active Directory site (Default-First-Site-Name)
But if you create a new site then your Exchange won’t know which site to look into , because of the not defined subnets
and Your services may stop and Restarting the server might struck in “Applying computer settings”
To avoid this situation,Before your Create a new Active Directory site
Define the Subnet for your your Default Active Directory site (Default-First-Site-Name)
Now you can type the specific subnet
Now you can overcome some downtime !!