Changelly allows you to buy any digital currency including buying Monero (XMR) with USD. All you need is to have a Credit Card for this. On another website, to buy Monero or coins other than BTC, you must buy BTC first and then convert it to the digital currency you want. This process is not difficult but due to multiple transactions, the website charges you double fees where the estimated time increases automatically. That's why we need a website like Changelly, where you can buy Monero or any digital coin in the first transaction directly with USD.