When to use byref argument in a function?

When I develop a component for Webform module. I used the "function hook_webform_component_info_alter(&$components) " to add a new component, but failed, instead, used the "function hook_webform_component_info()", it worked!

So I was completely confused when to use byref argument in a hook function. Because a byref parameter should be used as altering the existing content.

In other words, is there a general rule for using byref argument or not?

Thanks!