I've used them for four main reasons: 1) You're paying for something already going through the mail (e.g. sending a form to the government). You can usually s

I've used them for four main reasons: 1) You're paying for something already going through the mail (e.g. sending a form to the government). You can usually send a check, but some banks don't issue checkbooks anymore. 2) You're sending a payment to an individual online (e.g. eBay, Craigslist), and who won't accept online payments. Some merchants do accept PayPal, but raise the price to account for the merchant fee; a money order may be significantly cheaper for large purchases. If it's a scam, it also becomes mail fraud and you have the Postal Inspection Service on your side. 3) You need to make a deposit payment to someone like a landlord. They often won't accept checks, because they can bounce. Money orders and cashiers checks are sometimes the only accepted payment. Of the two, a money order is usually cheaper than a cashier's check.