Add Cc stands for Add Carbon Copy
Though the mail can be sent to many IDs using the "To" column by typing multiple entries separated by a comma; Multiple entries in Add Cc column does the same thing and is usually used to convey information for courtesy sake. A carbon copy of the document is sent to them in literal sense.
And again if you are sending to multiple IDs from both To and Add Cc column; all the recipients are capable of viewing who all has received the copy of the mail.
Here comes the role of Add Bcc - Add Blind Carbon Copy
If you do not want your recipients to know whom all the copy of the mail has been dispatched; then adding multiple ID's in this column is helpful.