Background checks are all about information. Information that helps you minimize risks and possible legal consequences for not doing required due diligence.
Many people tend to freely express themselves through social networks and sometimes do get carried away in what they may divulge about themselves, their jobs or former bosses. While a background check may indicate past criminal/violent/dishonest behavior that may show probability of repeating. A social networking site may display and give employers an insight into personal moral values as well as predictable future behavior.
It pays to gather as much information in order to minimize risk and protect person and property. Life insurance companies profit on their ability ofp redicting risk, why should you not?