To calculate your genetic ancestry, 23andme uses three basic populations: European, African, and Asian; and compares your DNA to populations on those continents. Note: Native American and Pacific Islander ancestry is classified as "Asian," as those populations were thought to have drifted from Asia to the east. On your Ancestry Graph, there are two segments represented for each chromosome, one segment that you inherited from each parent. If that segment was European in origin, it will be colored blue, if African, green, and if Asian, orange.
Why provide this information? It is known that certain genes control specific functions in your body. If you carry Europeans DNA on certain chromosomes, you may be more resistant or susceptible to some diseases. Likewise, Sickle-Cell anemia will be African in origin. Secondly, the most obvious reason is for genealogical purposes. Lots of Americans claim to have descended from a "Cherokee princess." Ancestry painting will show if you have inherited "Asian" DNA. Likewise, African-Americans can see how much European DNA they have inherited, and most Hispanics will have a high European percentage combined with the Native American "Asian."
An African-American's Ancestry Painting:
A child of an Asian mother and Italian father:
100 % European. No Cherokee princess. Boring........