If the program automatically responds to everyone on the list, thousands of uninterested people could be forced to read about your vacation schedule. If you've been working for more than a few years, you remember when electronic mail was quite unusual. Today it's commonplace, and it's changing the way we work. It can even affect us physically. I gained about 20 pounds in the two years after email was installed throughout my workplace, and I'm pretty sure it was because I had so much less running around to do.
Many systems make it easy to send mail out to everyone at the site or everyone at the company.
Others don't actually receive that much mail, but can't seem to handle it anyway.