Because it provides context and relevance for the person included as a reference.
Explanation:
A reference sheet makes it so that potential employers or others might have a reliable source of information about your past behavior, from people who have witnessed it first hand preferably. If I were to put "John Smith" as my reference, a potential employer would have no idea why is Mr. Smith a reliable source of information. If instead I put "John Smith, Sales Manager at Dunder Mifflin" and on my resume I have a previous job as a salesman at Dunder Mifflin, a potential employer would know Mr. Smith and I worked together and he was most likely my boss and therefore he would have valuable information to provide as a reference.
