They're are completely different in all ways. With different uses and benefits. For wider usage and acceptance, USD as more countries use it to pair against their currency which makes it much more in demand and in circulation. And of course, legitimately. Bitcoin is not yet accepted worldwide even though lots of people are using it.

But for traders, when BTC is going down, convert to USD to stabilize your funds and once BTC picks up, buy back so you can enjoy some profits and be safe