Adding Constraints Using Expressions
When entities have defined properties the values of those properties often vary by entity. You can use these to create more focused queries based on different values.
For example, you could have an entity type of "person" and properties for that entity type could include gender, age, or parental status. These properties would make it possible for you to run queries that look only for males, or for people in a certain age group, or for people who have at least one child. There are endless reasons for wanting to run a targeted query, and incorporating expressions that are based on properties into your query allows you to do that.