Although each human being is unique in an absolute sense, just as each cat, snowflake, mosquito, or leaf is an absolute singular unique individual, each object, whether human or not, has characteristics that it holds in common with other individuals. In fact, to say that a given entity is "human" is to place it in category, just as to say that an entity is a "cat" also places it in a category. In fact, for a description of an object to be meaningful, it must include general terms.
Imagine I were to describe person ABC you had not met by saying "ABC looks like XYX". If you have not met XYZ, this is useless information. If I say that ABC belongs to familiar categories to you such as "woman", "elderly", "Shia Muslim", "widow", "wealthy", etc, you get a much better picture of her. Also when we study people to predict, for example, their risk of getting certain diseases, how they are likely to vote, etc. we need to divide them into categories to use statistical data. Overgeneralizing, by assuming that people who belong to one category (e.g. Muslim) also belong to unrelated categories (terrorist, philanthropist) leads us to make bad judgements. Some people who belong to the category Muslim (and many who do not) are terrorists and some who belong to the category Muslim are philanthropists and some are not.