Email addresses + passwords + possibly other things as well.

Many people reuse the same email and password on other services/websites, so this is pretty valuable and sensitive information.

Assholes who just like kicking anthills, assholes who seek to get peoples' accounts for profit, and assholes who like kicking anthills for profit.

People use the same passwords for multiple things. If they have passwords for one site they can try it for multiple websites.

emails + password re-usage = profit