There will always be a Buffer register in USART. As the name suggests it provides a synchronization b/w a controller and other device. If we directky write something the issue of clocking speed occurs. That is the speed at which CPU operates will be pretty higher than other devices. Thus the chances of losing data is invetiable. So in USART the BAUD RATE will help in this synchronization and the data is stored into the buffer register and then to the other device with the BAUD RATE (which is bits per second).