Believe it or not, but the most common background search people conduct is on themselves. People want to know what information is publicly available for others to see. "Public records" are documents or pieces of information that are publicly available. This means that anyone can access the information if they try hard enough to locate it.
For example, if a marriage is "public", then there will be a record of it in the county courthouse where the marriage occurred. The same concept applies for arrest records, etc.
One can spend hours, even days visiting courthouses and other facilities to search for information, or they can simply do a thorough background check online. Whether you are searching for information regarding yourself, neighbors, family members, friends, or a new romantic partner, doing a background search online is an effective way to find out the information that you need or are just curious about. Besides doing criminal background checks to protect yourself or family members, many people conduct background checks out of mere curiousity.