Short answer: Default Web Site is included everytime the IIS feature is enabled on a server, so you don't need it. Simple stop it and then go on with your new web application using port 80.
While creating a DNS entry for a web application, you create the entry for the servers IP adress and port.
As an example, imagine that you have created two webapplications (intranet.example.com and contoso.example.com) using the same port and created a DNS entry for both of them. (Both will have the same IP and port.)
Now if you would try to access any of them using the adress assigned, IIS won't simply know to which web application your request should be sent to because both of the web applications uses the same IP and port.
If you would like to use multiple webapplication on the same port, use HostHeaders.
Configure a Host Header for a Web Site