It's virtually impossible today to buy a computer that doesn't have a socket for you to plug speakers or headphones into, so why complicate the situation with extra hardware? The simple answer is that the default onboard audio you get from your computer isn't the best quality.
Whatever kind of computer you use for work, whether it's a Mac or PC, a desktop or laptop or even something like an iPad Pro, it's likely that you've never even thought about adding a USB sound card to your setup.
