Suppose you're not operating on the default Joomla database (the one returned by `JFactory::getDbo()`) but on another one entirely? In theory, it is possible. In that case, you will want this new table object to use the same DBO that `$this` is using.
If you look at any similar table classes that are built in to Joomla, you will see this same kind of thing. So this is consistent with Joomla practices.