Some people have little to no background to work with regarding family history. It can be difficult, but not impossible to create a family map when working from scratch. DNA testing can provide starting points for those information seekers, and expand upon existing background for others. Simple tests can help to pinpoint specific nationalities, making it somewhat easier to know where to focus research efforts.
For example, if your DNA profile corroborate your family's origins, making a break in your case that much easier. If you get to the point where you're truly stumped you can always consult an expert genealogist or use a pay service that specializes in genetic research, like African Ancestry or World Vital Records.