Big companies contract models for their advertisement campaign exclusively, e.g. Cosmetic enterprises or designer labels. The advantage for those companies: Their clients identify the brand with a famous person, so it is a high recognition value for the client. The advantage for the model: A good payment.
Some agencies offer their models exclusive contracts (for their sole representation in a specific area). So, the model is very dependent on this one agency. So, you always have to make a decision if the agency is worth it or not.