to catch more 'variations' of the olldomomain hostname -- www- and non-www, FQDN, and appended ports, for example.
Note that the RewriteCond testing %{REQUEST_URI} is not needed -- the URL-path is already being tested by the rule itself.
This code in it's current form assumes that all domains are hosted in the same 'file space' so that this .htaccess file will be executed when a browser makes a request for "www.olddomain.com". If these old domains are hosted separately, then the remaining RewriteCond won't be needed either -- as long as the code is located in a .htaccess file that will be processed when that domain is requested.