Why do we do this and what information might we use to classify/categorize people?

We say that each human is unique yet we constantly classify and categorize individuals.  

Why do we do this and what information might we use to classify/categorize people? 

What are the advantages and disadvantages of categorizing individuals?  

(Hint: to answer this question think about the drawbacks of exaggerating within-group similarity, such as stereotyping).

reference: Reisberg, D. (2018). Cognition: Exploring the science of the mind (7th ed.). New York, NY: W. W. Norton & Company. ISBN: 978-0-393-69120-7