Interest is the price you pay for using someone else's money. Over long periods (like a 30-year mortgage), it is common to pay more in interest than the original value of the loan itself. This calculator helps you see the "true cost" of a purchase. That $30,000 car might actually cost you $38,000 once interest is factored in.
Extending a loan term lowers your monthly payment, but it drastically increases your lifetime interest cost. A 5-year car loan at 6% costs far less in total than a 7-year loan at the same rate, even though the 7-year loan has a "cheaper" monthly payment.