Some people have to buy Part A because they don't qualify for premium-free Part A. If you have to buy Part A, and you don't buy it when you're first eligible for Medicare, your monthly premium may go up 10% for each 12-month period you could've had Part A, but didn't sign up. You'll have to pay the higher premium for twice the number of years you didn't sign up.
If you have limited income and resources, your state may help you pay for Part A, and/or Part B. You may also qualify for Extra Help to pay for your Medicare prescription drug coverage.