Accessibility to the school. If you live on-campus, you can easily walk to classes, libraries, computer labs, cafeterias, etc. You don't have to waste time and money on driving to school, finding a parking space, and so forth.
Increased social life. Living on-campus will allow you to meet more people and establish more friendships than if you live off-campus. Living in a dorm means that there will always be someone around to hang out with. You will also usually be aware of any campus activities and how to be involved in them.
Simplicity. If you live on-campus, you don't have to worry about monthly rent payments, utility bills, and grocery shopping. Most students who live in a dorm get a meal plan in order to make eating easy and convenient. (Although, you will probably still want to dine in your dorm room from time to time.)