Aggregation is an abstraction that treats relationships as entities. Sometimes we have to model relationship between a collection of entities and relationships.
Consider the case where we have to carry out evaluation for a project for which Professor X is guiding students Y and Z.
We have a relationship called proj_guide which maps project, student, instructor entity sets. The evaluation is done for all i.e. the student, the instructor and the project. So basically the relationship eval_for maps the relationship proj_guide to the entity set evaluation. Using aggregation we can treat the proj_guide relation along with the instructor, student, project entity sets as a single entity set on which we can define a relationship called eval_for.
This is shown by enclosing the relationship and the corresponding entity sets in a box and then treating it as a single entity. In the first case, eval_for has been defined on the instructor, student and the project entities separately though it is really a characteristic of the three put together. Aggregation rectifies this.