Why call javascript behaviour within its own function?
You're reading that wrong - the return from once() isn't Drupal.behaviors (it's a collection of jQuery elements), and Drupal.behaviors.behaviourName isn't a function that will do anything by invoking it, anyway. You would need

Drupal.behaviors.behaviourName.attach();


To get anything meaningful, but that's not synonymous with the code you're referencing.

So that code is not calling a javascript behaviour within its own function. It's calling a jQuery extension function on the elements returned from once(). Most commonly when code needs to do what you're describing in Drupal, you'll see Drupal.attachBehaviors(); instead.

What you have is a convention used by the developer who wrote the script, nothing more; you'll need to ask them for their motivation for doing so, and how/where they're attaching the $.fn functions of the same names as the behaviours.