Have you ever received an email from an unfamiliar name but with a subject line you recognized? Chances are someone sent out an email to a lot of people, including you, but instead of addressing it using BCC (which is a blind copy), that person used either TO or CC. Some other recipient of that email wanted to respond to the sender, but instead of clicking on Reply clicked on Reply All. Because of that, every person on that list got an email that very likely they cared nothing about. We all get far too much email, and these annoying responses that do not concern us only add to the problem.
The other problem with using TO or CC instead of BCC is that it reveals your email address to many people, some of whom you might not want to have it. Unless every person receiving this email needs to see who else is getting it, do your readers a big favor and use BCC.