haven't had my DNA tested, but my understanding is that it gives links to other DNA groups in the world and shows your percentage of each--and may give you links to others with same info.
But with geneology, researchers don't just depend upon"family history" or tales handed down--way too many inaccuracies. When researching, you have to have solid proof--like census, birth & death certificates, land titles, marriage records, etc. And most of us want at least 2-3 items that back each other up on information. Obviously, if the information recorded has been a lie to start with (which has happened on birth certificates and even census records) then it could cause one's DNA to be in conflict with the geneology. Also, in researching, sometimes records have been totally lost--i.e. Irish birth/marriage/death certificates for a chunk of time; US military records for WWI when the archive in St. Louis burnt, etc. In the case of birth certificates, and even marriage records, when the person was not born in a location where record keeping was routine (i.e. our own Great Plains when pioneers migrated). Geneology research is fun but one must be done accurately. DNA is very helpful for those that have no idea about ancestry, or only have parts of family ancestry but want to see what else may be there. Also excellent for those wanting to check for info on DNA to certain groups that are more vulnerable to health risks.