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?

There are both positive and negative classifications for people. Those which are used to denigrate or put down others are negative, while those that categorize them by positive attributes or their choices are more positive.
Classification is necessary for gathering information and understanding trends, which is common in the social sciences. People act in certain ways depending on what group they belong to. For instance, bluegrass enthusiasts are less likely to attend a Kelly Clarkson concert than, say, pop music fans.
The advantage of classification is that it helps identify these trends and tendencies in populations. It also serves to simplify analysis of groups. However, on the other hand, it overlooks individual preferences and tendencies. Additionally, it can lead to stereotyping and negative associations.