In the ref document.
All processes begin execution in user mode, and they switch to kernel mode only when obtaining a service provided by the kernel.
Does it mean when user program needs to use IO devices then it require to switch the mode from user mode to kernel mode, and perform the required task.
Example, printf() function call needs Display device like monitor to show the output, therefore, it switch its mode to kernel mode and when the task happened, mode changes to user mode again. and whenever it needs IO in future again do the same process.