True, but it enables you to compare things against those which you can't buy upfront. London Travelcard zones 1-6 is £3264 annually or £313/month, but there's no option to buy multiple years in advance as if you were buying a car outright.

£313/mo is more expensive than the cheapest new car leases, but you wouldn't want to drive a car through London zone 1, and you'd have to pay £18/day in the congestion zone.