If we can already execute C programs on cortex-m like micro-controllers, Why do we even need to install RTOS (or other operating systems).? What benefits it can provide if micro-controller is intended to be multi-purpose.?
An operating system provides a level of abstraction between the code written by an application programmer and the actual hardware the program runs on. So you don't have to worry, as an application programmer, about the details of the hardware, as they are handled by drivers. And thus you can compile the same program for many different hardware platforms, if they run the same (or a compatible) operating system.