Financial aid covers so many things. Here is one example I just heard about that blows my mind:
During Winter Study (a month-long mini-term in January), skiing is being offered through the physical education department. Williams is covering all the costs for a financial aid student.
If you are paying full tuition, Williams costs a lot. But if you qualify for financial aid, they really take care of you and make sure you are not excluded from anything that is happening for other students on campus.