It is mostly used to get the information you need from related model so that it become 'available' to be used by the model that defines the related field.  One most common example I face is when that piece of information is needed for domain.

E.g. In Sale Order: I want the Invoice Address to have the same State as the Customer's Address.  No cross-state invoicing.  So, you define the related field to get Customer's Address' State and then use it to domain Invoice Address.