Healthcare in the U.S. is very expensive. According to a U.S. government website, if you break your leg, you could end up with a bill for $7,500. If you need to stay in the hospital for three days, it would probably cost about $30,000. If you have health insurance, your costs will be much, much less.
Visa regulations require that you have insurance. If you have a J-1 visa, the U.S. Department of State requires that you (and all your family members with J-2 visas) have health insurance that meets specific requirements. If you are buying insurance from MIT, you must buy it within 30 days of your arrival, and you must continue making regular payments to keep your insurance coverage for as long as you stay. The government also requires health insurance for U.S. citizens and legal residents.