Why do people need to buy 1K email adds



You can mass mail with gmail accounts (100 per box I think?) and being gmail there is a good chance they'll hit the inbox. It avoids the hassle with your isp (like comcast) or your hosting provider (who probably limits you per min/hour ect).
 
Gmails are much better for inboxing than using your own hosting account, and sometimes you cannot use some of the big name companies like Aweber if you're planning on sending out unsolicted emails.

You can automate sending through 1000's of Gmail accounts to get a mass mailing solution for free, that's one of the reason why people buy them in bulk.