Why check out elsewhere after import?

[Chris]

http://ximbiot.com/cvs/manual/cvs-1.11.6/cvs_16.html#SEC138 says:

   if you want to work with the sources import them first and then
   check them out into a different directory

But it's not clear to me why I'd do it in a _different_ directory.  Why
can't I just check it out where I am?  Do I need the untouched source
tree for later reimports of vendor revisions?