Why do job applications ask for your race? many of us have multiple races?

for example, i have a couple of different races, but i look more of one than the others.. i have Aryan, Mediterranean (spanish, french and italian), African, and two Native American (tribes or maybe more).. however for a job application i was told to say im black. why? i like my heritage in its totality. what is the point to selecting a race either way? isn't it against the constitution to discriminate? then way ask these if it does not matter? i don't get it. could it be a way to group people, one with someone alike. but then again isn't that segregation? throw what you know, open to anything. thanks