When sending an email:
‘To’ is for action
If you want someone to do something, then you send the email to them.
‘CC’ is for information
If there is something in the email you are sending that you think someone should know, but they don’t have to do anything with that information, CC them.
If the reply is likely to contain some information that you think someone should know, then CC them and hope the person replying clicks ‘Reply-all’.