When sending an email, you probably have noticed three places you can list the recipient's email address. To, CC, and BCC. Of course, everyone knows the To is for sending to a single recipient. But what if you want to send to multiple recipients? Don't want to take the time to spend an individual email to each. That is what CC and BCC is for.
CC is carbon copy. BCC is blind carbon copy. Basically, in each case, a copy of your email is sent to each recipient. The difference: With CC everyone you send the email to gets the email address of everyone else. With BCC no one's email address is shown.
Have no idea why CC is even an option. Always use BCC. Most importantly, most people do not want their email address to be given to everyone on your email list. Think of it this way: Using CC is like sending a physical letter to several people and including a copy of every one's mailing addresses. Do not assume they are all friends with each other and already had each other's addresses.
Even if you know for a fact that everyone you are sending the email to has each other's address, use BCC. Ever get a mass email sent CC? Annoying to scroll through every one's email to get to the text. And if you go to print... I suppose some email programs might work differently, but I have gone to print off mass CC emails with five lines of text and ended up with two printed pages. Yes, I could take steps to make sure just the text gets printed, but that means extra steps. And I do not print off such emails often enough to remember to take those steps in time. Usually remember part way through the print job.