First, there are the users who have no idea that the ‘Cc’ exists. Every address is listed in the ‘To’ even if the email is only directed to one person. In cases such as this the receivers have no clue as to who should take action so either they all do something or they all do nothing.
Secondly, there are users who feel that every single e-mail should be copied to their entire address book whether it’s relevant to those receiving it or not. These are the ‘cry for attention’ crowd.
Lastly, there are users who never read the names of the people who receive a copy of an e-mail. They are the ‘Did you see this?’ crowd. For example, person X sends an e-mail to persons A, B and C. C immediately forwards it to A and B with the question ‘Did you see this?’ not bothering to see that X already sent A and B copies.
The addresses in the ‘To’ are for the people you are directly addressing. The addresses in the ‘Cc’ are for the people you are indirectly addressing. Copy only those who need to be copied; not your entire universe of contacts. The addresses in the ‘Bcc’ are like ‘Cc’ except that the addresses in ‘To’ and ‘Cc’ do not know that the addresses in the ‘Bcc’ are included in the conversation. The ‘To’ and ‘Cc’ addresses are blind to the ‘Bcc’ addresses.
Quite often* when I send an email to a bunch of people I’ll put everyone in the BCC, because some of my friends might not want their email spread around to people they don’t know, even if they do have me in common.
*And other times I forget.