A lot of campuses have actual police. In some ways campuses are similar to a municipality and have many of the services that entails (fire, trash, water, power, snow removal, parks, and security/law enforcement).

You could similarly ask why a town of 50,000 people needs a police department.

My university had a police department because it is, as one might expect of a flagship state university, an enormous landmass with billions of dollars in infrastructure and equipment, plus about 6,000 full-time residents living in university-owned housing. Residents of a certain class of people who are prone to doing stupid, illegal shit.

Why?