Question

Why would you copy an individual instead of listing that name on the To line?

Answer 1

When you send a letter or an email to somebody in the course
of your job, you might copy your boss. Even though he really
doesn't need the information in your correspondence, and you
don't expect him to answer you or take any action, you still want
him to know that you're doing your job, and how you're handling
this matter, and you want him to have the information available
IF he wants to see it.

Answer 2

When the person is not directly involved in the message but has a need to know about it.

Step-by-step explanation

Even though the person really doesn't need the information in your correspondence, and you don't expect him to answer you or take any action, you still want him to know that you're doing your job, and how you're handling this matter, and you want him to have the information available if he wants to see it.