Setting the baud rate of USART in STM32F103RB.

Why do we need to calculate the baudrate using the formula,

baud =fCK / (16*USARTDIV)

I mean why can't we write 9600 or any other desired baudrate directly in USART_BRR register? What is the purpose of performing  this calculation first? What are we calculating here anyway?

This might be a dumb question to ask, but I am really confused here. Please help.

Thanks in advance!