One common reason why people use genomic tests is to understand their ethnic heritage. They want to know where their ancestors come from, and genetic testing kits are able to give them a great picture of where they come from.
Y Chromosomes (chromosomes inherited by sons from their fathers) can be used to explore the male line of the family. This might be used to explore how related two families are with the same last name.
Mitochondrial DNA is used to learn more about the genome which is inherited from direct female ancestors because it is passed down.
Genetic testing for determining ancestry looks at data which is gathered from other samples in order to determine similarities between those samples. It comes with limitations, but it’s also a great place to start if you’re looking to gather more information about your past.