Though most computers since mid-2004 can boot from USB mass storage devices, USB is not intended as a primary bus for a computer's internal storage. Buses such as Parallel ATA (PATA or IDE), Serial ATA (SATA), or SCSI fulfill that role in PC class computers. However, USB has one important advantage, in that <answer>it is possible to install and remove devices without rebooting the computer</answer> (hot-swapping), making it useful for mobile peripherals, including drives of various kinds (given SATA or SCSI devices may or may not support hot-swapping).
What is an important advantage of USB?
it is possible to install and remove devices without rebooting the computer