Why RTOS?
Most microcontrollers developers writing sequential code for their applications. The microcontroller will execute each function called and asked what to do. This is good for small applications where the microcontroller is supposed to do only one thing in particular. Say you want to use your microcontroller to flash an LED and nothing else or flash an LED and then display a string on the LCD display, you'd do well without feeling the need to implement a system operating within your microcontroller. But for other applications where more than one thing is supposed to be done at the same time, you're stuck.
