It does have one benefit. www.google.com has its IP served from google's dns servers, whereas google.com comes from com's. Google does lots of dns-based load balancing that com doesn't support.
com. only serves the nameserver (NS) addresses for example.com., and if they are within the same domain, their IP addresses. It does not serve the rest of the records for example.com.
You can see this easily with e.g.:
dig -t any google.com. @a.gtld-servers.net.
vs.
dig -t any google.com. @ns1.google.com.
There is no real advantage of having it be 'www' coming from DNS...
There is a somewhat more modern problem which is that having a domain be a CNAME (the DNS equivalent of a by-name redirect) means it can't also have MX records and receive mail; the CNAME also ends up "redirecting" everything, including MX lookups. So if your web host offers yourdomain.someawesomehost.com and you decide to point yourdomain.com to it via CNAME, having you@yourdomain.com doesn't work unless yourdomain.someawesomehost.com is configured to receive mail for yourdomain.com.
