What are the advantages of CNAMEs?
CNAMEs are great timesavers. Instead of having to create multiple hostnames that contain the same information, you can create one hostname and then create and attach multiple CNAMEs to that hostname.

Let's say you want to create a number of hostnames with the same web presence as www.my-example.com. Instead of creating separate hostnames and setting up a web presence for each, you can create CNAMEs for my-example.com and attach them to the www.my-example.com hostname. Any changes you make to the www hostname will automatically change all the attached CNAMEs accordingly. This saves you the trouble of modifying many hostnames individually if, for example, your website address changed.

Do CNAMEs have to be attached to hostnames within the same domain?
CNAMEs can be attached to hostnames within the same domain but they don't have to be. For example, you can create the CNAME cname1.my-example.com and attach it to a hostname created from the same my-example.com domain: www.my-example.com. However, you can also create the CNAME cname2.my-example.com and attach it to a hostname of another domain such as www.mydomain.com or any other hostname on the Internet.

In each of these cases the CNAME that is created will act exactly like the hostname it is attached to. When entered as a website address the CNAME will direct visitors to the website address set for the hostname. And if there is an MX record set for the hostname, then email addressed to the CNAME will be delivered to the same mail server set for the hostname.