Collocation assists in making an essay/text predictable and easy to follow.
There are many types such as:
Discussing attributes of a person/object or abstract ideas
Relating to a subject as a whole or part such as body parts of a bird - wings, beak or perhaps discussing a character.
He was envious of her looks because his reflection was looked at as disgust.
collocation is envious, reflection.
You could also generalise on a topic and use collocation to enhance cohesion of the text. For example, I have read a story about a creek. The word collocation is: native flora and fauna, waste products, plastic bags, introduced species...
These are all associated with each other in the story.
Hope this helps